Supercapacitor having highly conductive graphene foam electrode

ABSTRACT

A supercapacitor electrode comprising a solid graphene foam impregnated with a liquid or gel electrolyte, wherein the solid graphene foam is composed of multiple pores and pore walls, wherein pore walls contain a pristine graphene material having essentially zero % of non-carbon elements, or a non-pristine graphene material having 0.001% to 5% by weight of non-carbon elements wherein non-pristine graphene is selected from graphene oxide, reduced graphene oxide, graphene fluoride, graphene chloride, graphene bromide, graphene iodide, hydrogenated graphene, nitrogenated graphene, chemically functionalized graphene, or a combination thereof, and the solid graphene foam, when measured in a dried state without electrolyte, has a physical density from 0.01 to 1.7 g/cm 3 , a specific surface area from 50 to 3,200 m 2 /g, a thermal conductivity of at least 200 W/mK per unit of specific gravity, and/or an electrical conductivity no less than 2,000 S/cm per unit of specific gravity.

FIELD OF THE INVENTION

The present invention relates generally to the field of supercapacitorand, more particularly, to a graphene foam-based electrode, asupercapacitor containing such an electrode, and a process for producingsame.

BACKGROUND OF THE INVENTION A Critical Review on Supercapacitors

Electrochemical capacitors (ECs), also known as ultracapacitors orsupercapacitors, are being considered for uses in hybrid electricvehicles (EVs) where they can supplement a battery used in an electriccar to provide bursts of power needed for rapid acceleration, thebiggest technical hurdle to making battery-powered cars commerciallyviable. A battery would still be used for cruising, but supercapacitors(with their ability to release energy much more quickly than batteries)would kick in whenever the car needs to accelerate for merging, passing,emergency maneuvers, and hill climbing. The EC must also storesufficient energy to provide an acceptable driving range. To be cost-,volume-, and weight-effective compared to additional battery capacitythey must combine adequate energy densities (volumetric and gravimetric)and power densities with long cycle life, and meet cost targets as well.

ECs are also gaining acceptance in the electronics industry as systemdesigners become familiar with their attributes and benefits. ECs wereoriginally developed to provide large bursts of driving energy fororbital lasers. In complementary metal oxide semiconductor (CMOS) memorybackup applications, for instance, a one-Farad EC having a volume ofonly one-half cubic inch can replace nickel-cadmium or lithium batteriesand provide backup power for months. For a given applied voltage, thestored energy in an EC associated with a given charge is half thatstorable in a corresponding battery system for passage of the samecharge. Nevertheless, ECs are extremely attractive power sources.Compared with batteries, they require no maintenance, offer much highercycle-life, require a very simple charging circuit, experience no“memory effect,” and are generally much safer. Physical rather thanchemical energy storage is the key reason for their safe operation andextraordinarily high cycle-life. Perhaps most importantly, capacitorsoffer higher power density than batteries.

The high volumetric capacitance density of an EC relative toconventional capacitors (10 to 100 times greater than conventionalcapacitors) derives from using porous electrodes to create a largeeffective “plate area” and from storing energy in the diffuse doublelayer. This double layer, created naturally at a solid-electrolyteinterface when voltage is imposed, has a thickness of only about 1 nm,thus forming an extremely small effective “plate separation.” Such asupercapacitor is commonly referred to as an electric double layercapacitor (EDLC). The double layer capacitor is based on a high surfacearea electrode material, such as activated carbon, immersed in a liquidelectrolyte. A polarized double layer is formed at electrode-electrolyteinterfaces providing high capacitance. This implies that the specificcapacitance of a supercapacitor is directly proportional to the specificsurface area of the electrode material. This surface area must beaccessible by electrolyte and the resulting interfacial zones must besufficiently large to accommodate the so-called electric double-layercharges.

In some ECs, stored energy is further augmented by pseudo-capacitanceeffects, occurring again at the solid-electrolyte interface due toelectrochemical phenomena such as the redox charge transfer.

However, there are several serious technical issues associated withcurrent state-of-the-art ECs or supercapacitors:

-   (1) Experience with ECs based on activated carbon electrodes shows    that the experimentally measured capacitance is always much lower    than the geometrical capacitance calculated from the measured    surface area and the width of the dipole layer. For very high    surface area activated carbons, typically only about 20 percent of    the “theoretical” capacitance was observed. This disappointing    performance is related to the presence of micro-pores (<2 nm, mostly    <1 nm) and ascribed to inaccessibility of some pores by the    electrolyte, wetting deficiencies, and/or the inability of a double    layer to form successfully in pores in which the oppositely charged    surfaces are less than about 1-2 nm apart. In activated carbons,    depending on the source of the carbon and the heat treatment    temperature, a surprising amount of surfaces can be in the form of    such micro-pores that are not accessible to liquid electrolyte.-   (2) Despite the high gravimetric capacitances at the electrode level    (based on active material weights alone) as frequently claimed in    open literature and patent documents, these electrodes unfortunately    fail to provide energy storage devices with high capacities at the    supercapacitor cell or pack level (based on the total cell weight or    pack weight). This is due to the notion that, in these reports, the    actual mass loadings of the electrodes and the apparent densities    for the active materials are too low. In most cases, the active    material mass loadings of the electrodes (areal density) is    significantly lower than 10 mg/cm² (areal density=the amount of    active materials per electrode cross-sectional area along the    electrode thickness direction) and the apparent volume density or    tap density of the active material is typically less than 0.75    g/cm⁻³ (more typically less than 0.5 g/cm⁻³ and most typically less    than 0.3 g/cm⁻³) even for relatively large particles of activated    carbon.

The low mass loading is primarily due to the inability to obtain thickerelectrodes (thicker than 100-200 μm) using the conventional slurrycoating procedure. This is not a trivial task as one might think, and inreality the electrode thickness is not a design parameter that can bearbitrarily and freely varied for the purpose of optimizing the cellperformance. Contrarily, thicker samples tend to become extremelybrittle or of poor structural integrity and would also require the useof large amounts of binder resin. These problems are particularly acutefor graphene material-based electrodes. It has not been previouslypossible to produce graphene-based electrodes that are thicker than 100μm and remain highly porous with pores remaining fully accessible toliquid electrolyte. The low areal densities and low volume densities(related to thin electrodes and poor packing density) result inrelatively low volumetric capacitances and low volumetric energy densityof the supercapacitor cells.

With the growing demand for more compact and portable energy storagesystems, there is keen interest to increase the utilization of thevolume of the energy storage devices. Novel electrode materials anddesigns that enable high volumetric capacitances and high mass loadingsare essential to achieving improved cell volumetric capacitances andenergy densities.

-   (3) During the past decade, much work has been conducted to develop    electrode materials with increased volumetric capacitances utilizing    porous carbon-based materials, such as graphene, carbon    nanotube-based composites, porous graphite oxide, and porous meso    carbon. Although these experimental supercapacitors featuring such    electrode materials can be charged and discharged at high rates and    also exhibit large volumetric electrode capacitances (100 to 200    F/cm³ in most cases, based on the electrode volume), their typical    active mass loading of <1 mg/cm², tap density of <0.2 g/cm⁻³, and    electrode thicknesses of up to tens of micrometers (<<100 μm) are    still significantly lower than those used in most commercially    available electrochemical capacitors (i.e. 10 mg/cm², 100-200 μm),    which results in energy storage devices with relatively low areal    and volumetric capacitances and low volumetric energy densities.-   (4) For graphene-based supercapacitors, there are additional    problems that remain to be solved, explained below:

Nano graphene materials have recently been found to exhibitexceptionally high thermal conductivity, high electrical conductivity,and high strength. Another outstanding characteristic of graphene is itsexceptionally high specific surface area. A single graphene sheetprovides a specific external surface area of approximately 2,675 m²/g(that is accessible by liquid electrolyte), as opposed to the exteriorsurface area of approximately 1,300 m²/g provided by a correspondingsingle-wall CNT (interior surface not accessible by electrolyte). Theelectrical conductivity of graphene is slightly higher than that ofCNTs.

The instant applicants (A. Zhamu and B. Z. Jang) and their colleagueswere the first to investigate graphene- and other nano graphite-basednano materials for supercapacitor application [Please see Refs. 1-5below; the 1^(st) patent application was submitted in 2006 and issued in2009]. After 2008, researchers began to realize the significance ofgraphene materials for supercapacitor applications.

LIST OF REFERENCES

-   -   1. Lulu Song, A. Zhamu, Jiusheng Guo, and B. Z. Jang        “Nano-scaled Graphene Plate Nanocomposites for Supercapacitor        Electrodes” U.S. Pat. No. 7,623,340 (Nov. 24, 2009).    -   2. Aruna Zhamu and Bor Z. Jang, “Process for Producing        Nano-scaled Graphene Platelet Nanocomposite Electrodes for        Supercapacitors,” U.S. patent application Ser. No. 11/906,786        (Oct. 4, 2007).    -   3. Aruna Zhamu and Bor Z. Jang, “Graphite-Carbon Composite        Electrodes for Supercapacitors” U.S. patent application Ser. No.        11/895,657 (Aug. 27, 2007).    -   4. Aruna Zhamu and Bor Z. Jang, “Method of Producing        Graphite-Carbon Composite Electrodes for Supercapacitors” U.S.        patent application Ser. No. 11/895,588 (Aug. 27, 2007).    -   5. Aruna Zhamu and Bor Z. Jang, “Graphene Nanocomposites for        Electrochemical cell Electrodes,” U.S. patent application Ser.        No. 12/220,651 (Jul. 28, 2008).

However, individual nano graphene sheets have a great tendency tore-stack themselves, effectively reducing the specific surface areasthat are accessible by the electrolyte in a supercapacitor electrode.The significance of this graphene sheet overlap issue may be illustratedas follows: For a nano graphene platelet with dimensions of l (length)×w(width)×t (thickness) and density ρ, the estimated surface area per unitmass is S/m=(2/ρ)(1/l+1/w+1/t). With ρ≅2.2 g/cm³, l=100 nm, w=100 nm,and t=0.34 nm (single layer), we have an impressive S/m value of 2,675m²/g, which is much greater than that of most commercially availablecarbon black or activated carbon materials used in the state-of-the-artsupercapacitor. If two single-layer graphene sheets stack to form adouble-layer graphene, the specific surface area is reduced to 1,345m²/g. For a three-layer graphene, t=1 nm, we have S/m=906 m²/g. If morelayers are stacked together, the specific surface area would be furthersignificantly reduced.

These calculations suggest that it is critically important to find a wayto prevent individual graphene sheets from re-stacking and, even if theypartially re-stack, the resulting multi-layer structure would still haveinter-layer pores of adequate sizes. These pores must be sufficientlylarge to allow for accessibility by the electrolyte and to enable theformation of electric double-layer charges, which presumably require apore size of at least 1-2 nm. However, these pores or inter-graphenespacings must also be sufficiently small to ensure a large tap density.Unfortunately, the typical tap density of graphene-based electrodeproduced by the conventional process is less than 0.3 g/cm³, and mosttypically <<0.2 g/cm³. To a great extent, the requirement to have largepore sizes and high porosity level and the requirement to have a hightap density are considered mutually exclusive in supercapacitors.

Another major technical barrier to using graphene sheets as asupercapacitor electrode active material is the challenge of forming athick active material layer onto the surface of a solid currentcollector (e.g. Al foil) using the conventional graphene-solvent slurrycoating process. In such an electrode, the graphene electrode typicallyrequires a large amount of a binder resin (hence, significantly reducedactive material proportion vs. non-active or overheadmaterials/components). In addition, any electrode prepared in thismanner that is thicker than 50 μm is brittle and weak. There has been noeffective solution to these problems.

Therefore, there is clear and urgent need for supercapacitors that havehigh active material mass loading (high areal density), active materialswith high apparent density (high tap density), high electrode thicknesswithout significantly decreasing the electron and ion transport rates(e.g. without a long electron transport distance), high volumetriccapacitance, and high volumetric energy density. For graphene-basedelectrodes, one must also overcome problems such as re-stacking ofgraphene sheets, the demand for large proportion of a binder resin, anddifficulty in producing thick graphene electrode layers.

Carbon is known to have five unique crystalline structures, includingdiamond, fullerene (0-D nano graphitic material), carbon nano-tube orcarbon nano-fiber (1-D nano graphitic material), graphene (2-D nanographitic material), and graphite (3-D graphitic material). The carbonnano-tube (CNT) refers to a tubular structure grown with a single wallor multi-wall. Carbon nano-tubes (CNTs) and carbon nano-fibers (CNFs)have a diameter on the order of a few nanometers to a few hundrednanometers. Their longitudinal, hollow structures impart uniquemechanical, electrical and chemical properties to the material. The CNTor CNF is a one-dimensional nano carbon or 1-D nano graphite material.

Our research group was among the first to discover graphene [B. Z. Jangand W. C. Huang, “Nano-scaled Graphene Plates,” U.S. patent applicationSer. No. 10/274,473, submitted on Oct. 21, 2002; now U.S. Pat. No.7,071,258 (Jul. 4, 2006)]. The processes for producing NGPs and NGPnanocomposites were recently reviewed by us [Bor Z. Jang and A Zhamu,“Processing of Nano Graphene Platelets (NGPs) and NGP Nanocomposites: AReview,” J. Materials Sci. 43 (2008) 5092-5101].

For the purpose of defining the claims of the instant application, NGPsor graphene materials include discrete sheets/platelets of single-layerand multi-layer (typically less than 10 layers) pristine graphene,graphene oxide, reduced graphene oxide (RGO), graphene fluoride,graphene chloride, graphene bromide, graphene iodide, hydrogenatedgraphene, nitrogenated graphene, chemically functionalized graphene,doped graphene (e.g. doped by B or N). Pristine graphene has essentially0% oxygen. RGO typically has an oxygen content of 0.001%-5% by weight.Graphene oxide (including RGO) can have 0.001%-50% by weight of oxygen.Other than pristine graphene, all the graphene materials have 0.001%-50%by weight of non-carbon elements (e.g. O, H, N, B, F, Cl, Br, I, etc.).These materials are herein referred to as non-pristine graphenematerials. The presently invented graphene-carbon foam can containpristine or non-pristine graphene and the invented method allows forthis flexibility.

A Review on Production of Graphene Foams

Generally speaking, a foam or foamed material is composed of pores (orcells) and pore walls (a solid material). The pores can beinterconnected to form an open-cell foam. A graphene foam is composed ofpores and pore walls that contain a graphene material. There are threemajor methods of producing graphene foams:

The first method is the hydrothermal reduction of graphene oxidehydrogel that typically involves sealing graphene oxide (GO) aqueoussuspension in a high-pressure autoclave and heating the GO suspensionunder a high pressure (tens or hundreds of atm) at a temperaturetypically in the range of 180-300° C. for an extended period of time(typically 12-36 hours). A useful reference for this method is givenhere: Y. Xu, et al. “Self-Assembled Graphene Hydrogel via a One-StepHydrothermal Process,” ACS Nano 2010, 4, 4324-4330. There are severalmajor issues associated with this method: (a) The high pressurerequirement makes it an impractical method for industrial-scaleproduction. For one thing, this process cannot be conducted on acontinuous basis. (b) It is difficult, if not impossible, to exercisecontrol over the pore size and the porosity level of the resultingporous structure. (c) There is no flexibility in terms of varying theshape and size of the resulting reduced graphene oxide (RGO) material(e.g. it cannot be made into a film shape). (d) The method involves theuse of an ultra-low concentration of GO suspended in water (e.g. 2mg/mL=2 g/L=2 kg/kL). With the removal of non-carbon elements (up to50%), one can only produce less than 2 kg of graphene material (RGO) per1000-liter suspension. Furthermore, it is practically impossible tooperate a 1000-liter reactor that has to withstand the conditions of ahigh temperature and a high pressure. Clearly, this is not a scalableprocess for mass production of porous graphene structures.

The second method is based on a template-assisted catalytic CVD process,which involves CVD deposition of graphene on a sacrificial template(e.g. Ni foam). The graphene material conforms to the shape anddimensions of the Ni foam structure. The Ni foam is then etched awayusing an etching agent, leaving behind a monolith of graphene skeletonthat is essentially an open-cell foam. A useful reference for thismethod is given here: Zongping Chen, et al., “Three-dimensional flexibleand conductive interconnected graphene networks grown by chemical vapourdeposition,” Nature Materials, 10 (June 2011) 424-428. There are severalproblems associated with such a process: (a) the catalytic CVD isintrinsically a very slow, highly energy-intensive, and expensiveprocess; (b) the etching agent is typically a highly undesirablechemical and the resulting Ni-containing etching solution is a source ofpollution. It is very difficult and expensive to recover or recycle thedissolved Ni metal from the etchant solution. (c) It is challenging tomaintain the shape and dimensions of the graphene foam without damagingthe cell walls when the Ni foam is being etched away. The resultinggraphene foam is typically very brittle and fragile. (d) The transportof the CVD precursor gas (e.g. hydrocarbon) into the interior of a metalfoam can be difficult, resulting in a non-uniform structure, sincecertain spots inside the sacrificial metal foam may not be accessible tothe CVD precursor gas.

The third method of producing graphene foam also makes use of asacrificial material (e.g. colloidal polystyrene particles, PS) that iscoated with graphene oxide sheets using a self-assembly approach. Forinstance, Choi, et al. prepared chemically modified graphene (CMG) paperin two steps: fabrication of free-standing PS/CMG films by vacuumfiltration of a mixed aqueous colloidal suspension of CMG and PS (2.0 μmPS spheres), followed by removal of PS beads to generate 3D macro-pores.[B. G. Choi, et al., “3D Macroporous Graphene Frameworks forSupercapacitors with High Energy and Power Densities,” ACS Nano, 6(2012) 4020-4028.] Choi, et al. fabricated well-ordered free-standingPS/CMG paper by filtration, which began with separately preparing anegatively charged CMG colloidal and a positively charged PS suspension.A mixture of CMG colloidal and PS suspension was dispersed in solutionunder controlled pH (=2), where the two compounds had the same surfacecharges (zeta potential values of +13±2.4 mV for CMG and +68±5.6 mV forPS). When the pH was raised to 6, CMGs (zeta potential=−29±3.7 mV) andPS spheres (zeta potential=+51±2.5 mV) were assembled due to theelectrostatic interactions and hydrophobic characteristics between them,and these were subsequently integrated into PS/CMG composite paperthrough a filtering process. This method also has several shortcomings:(a) This method requires very tedious chemical treatments of bothgraphene oxide and PS particles. (b) The removal of PS by toluene alsoleads to weakened macro-porous structures. (c) Toluene is a highlyregulated chemical and must be treated with extreme caution. (d) Thepore sizes are typically excessively big (e.g. several μm), too big formany useful applications.

The above discussion clearly indicates that every prior art method orprocess for producing graphene foams has major deficiencies. Thus, it isan object of the present invention to provide a cost-effective processfor producing highly conductive, mechanically robust graphene-basedfoams (specifically, integral 3D graphene foam as a supercapacitorelectrode) in large quantities. This process enables the flexible designand control of the porosity level and pore sizes.

It is another object of the present invention to provide a process forproducing graphene foam-based supercapacitor electrode that exhibit athermal conductivity, electrical conductivity, elastic modulus, and/orstrength that are comparable to or greater than those of theconventional graphite or carbon foam-based electrode.

Yet another object of the present invention is to provide asupercapacitor electrode based on (a) a pristine graphene foam thatcontains essentially all carbon only and preferably have a pore sizerange of 0.5-50 nm); and (b) non-pristine graphene foams (graphenefluoride, graphene chloride, nitrogenated graphene, etc.) that containsat least 0.001% by weight (typically from 0.01% to 25% by weight andmost typically from 0.1% to 20%) of non-carbon elements.

Another object of the present invention is to provide a supercapacitorthat contains a graphene foam-based electrode of the present invention.

SUMMARY OF THE INVENTION

The present invention provides a supercapacitor electrode containing asolid graphene foam as an electrode active material. The graphene foamis pre-impregnated with a liquid or gel electrolyte. The liquidelectrolyte may contain an aqueous electrolyte, organic electrolyte,ionic liquid electrolyte, or a mixture of an organic and an ionic liquidelectrolyte.

The solid graphene foam is composed of multiple pores and pore walls,wherein the pore walls contain a pristine graphene material havingessentially zero % of non-carbon elements, or a non-pristine graphenematerial having from 0.001% to 5% by weight of non-carbon elementswherein the non-pristine graphene is selected from graphene oxide,reduced graphene oxide, graphene fluoride, graphene chloride, graphenebromide, graphene iodide, hydrogenated graphene, nitrogenated graphene,chemically functionalized graphene, doped graphene, or a combinationthereof, and the solid graphene foam has a density from 0.01 to 1.7g/cm³, a specific surface area from 50 to 3,300 m²/g, a thermalconductivity of at least 200 W/mK per unit of specific gravity, and/oran electrical conductivity no less than 2,000 S/cm per unit of specificgravity. In a preferred embodiment, the pore walls contain stackedgraphene planes having an inter-plane spacing d₀₀₂ from 0.3354 nm to0.36 nm as measured by X-ray diffraction.

In an embodiment, the pore walls contain a pristine graphene and thesolid graphene foam has a density from 0.1 to 1.7 g/cm³ or an averagepore size from 0.5 nm to 50 nm. In an embodiment, the pore walls containa non-pristine graphene material selected from the group consisting ofgraphene oxide, reduced graphene oxide, graphene fluoride, graphenechloride, graphene bromide, graphene iodide, hydrogenated graphene,nitrogenated graphene, chemically functionalized graphene, andcombinations thereof, and wherein the solid graphene foam contains acontent of non-carbon elements in the range of 0.01% to 2.0% by weight.In other words, the non-carbon elements can include an element selectedfrom oxygen, fluorine, chlorine, bromine, iodine, nitrogen, hydrogen, orboron. In a specific embodiment, the pore walls contain graphenefluoride and the solid graphene foam contains a fluorine content from0.01% to 2.0% by weight. In another embodiment, the pore walls containgraphene oxide and said solid graphene foam contains an oxygen contentfrom 0.01% to 2.0% by weight. In an embodiment, the solid graphene foamhas a specific surface area from 200 to 2,800 m²/g or a density from 0.1to 1.5 g/cm³. The non-carbon elements include an element selected fromoxygen, fluorine, chlorine, bromine, iodine, nitrogen, hydrogen, orboron.

In a preferred embodiment, the solid graphene foam is made into acontinuous-length roll sheet form (a roll of a continuous foam sheet)having a thickness from 10 nm to 10 mm and a length of at least 1 meterlong, preferably at least 2 meters, further preferably at least 10meters, and most preferably at least 100 meters. This sheet roll isproduced by a roll-to-roll process. There has been no prior art graphenefoam that is made into a sheet roll form. It has not been previouslyfound or suggested possible to have a roll-to-roll process for producinga continuous length of graphene foam, either pristine or non-pristine.

In certain desired embodiments, the solid graphene foam further containsa carbon or graphite material selected from carbon nanotubes, carbonnano-fibers, carbon fiber segments, graphite fiber segments, activatedcarbon, carbon black particles, carbon wires, natural graphiteparticles, needle coke particles, meso-carbon micro-beads, particles ofa natural or artificial graphite, expanded graphite flakes, or acombination thereof. These carbon or graphite materials can be easilyincorporated into the graphene suspension prior to a coating procedure.

In order to increase the specific capacitance and specific energy of thesupercapacitor, the multiple pores may contain a redox pair partnerselected from an intrinsically conductive polymer, a transition metaloxide, and/or an organic molecule, wherein said redox pair partner is inphysical or electronic contact with said graphene material, forming aredox pair therewith. The intrinsically conducting polymer may beselected from polyaniline, polypyrrole, polythiophene, polyfuran,sulfonated polyaniline, sulfonated polypyrrole, sulfonatedpolythiophene, sulfonated polyfuran, sulfonated polyacetylene, or acombination thereof.

In a preferred embodiment, the graphene foam has an oxygen content ornon-carbon content less than 1% by weight, and the pore walls havestacked graphene planes having an inter-graphene spacing less than 0.35nm, a thermal conductivity of at least 250 W/mK per unit of specificgravity, and/or an electrical conductivity no less than 2,500 S/cm perunit of specific gravity. In a further preferred embodiment, thegraphene foam has an oxygen content or non-carbon content less than0.01% by weight and said pore walls contain stacked graphene planeshaving an inter-graphene spacing less than 0.34 nm, a thermalconductivity of at least 300 W/mK per unit of specific gravity, and/oran electrical conductivity no less than 3,000 S/cm per unit of specificgravity.

In yet another preferred embodiment, the graphene foam has an oxygencontent or non-carbon content no greater than 0.01% by weight and saidpore walls contain stacked graphene planes having an inter-graphenespacing less than 0.336 nm, a mosaic spread value no greater than 0.7, athermal conductivity of at least 350 W/mK per unit of specific gravity,and/or an electrical conductivity no less than 3,500 S/cm per unit ofspecific gravity. In still another preferred embodiment, the graphenefoam has pore walls containing stacked graphene planes having aninter-graphene spacing less than 0.336 nm, a mosaic spread value nogreater than 0.4, a thermal conductivity greater than 400 W/mK per unitof specific gravity, and/or an electrical conductivity greater than4,000 S/cm per unit of specific gravity.

In a preferred embodiment, the pore walls contain stacked grapheneplanes having an inter-graphene spacing less than 0.337 nm and a mosaicspread value less than 1.0. In a preferred embodiment, the graphene foamexhibits a degree of graphitization no less than 80% (preferably no lessthan 90%) and/or a mosaic spread value less than 0.4. In a preferredembodiment, the pore walls contain a 3D network of interconnectedgraphene planes.

In a preferred embodiment, the solid graphene foam contains pores havingan average pore size from 0.5 nm to 50 nm. The solid graphene foam canalso be made to contain micron-scaled pores (1-500 μm).

The present invention also provides a supercapacitor comprising ananode, a porous separator-electrolyte layer or electrolyte-permeablemembrane, and a cathode, wherein either or both of the anode and thecathode contains the presently invented electrode. If both the anode andthe cathode contain such an electrode and the two electrodes haveidentical compositions, we have a symmetric supercapacitor. If theelectrode contains only a graphene material or a graphene and a carbonor graphite material as the only electrode active material, we have anelectric double layer capacitor (EDLC). The presently inventedsupercapacitor electrode is capable of delivering a specific capacitanceof 150-350 F/g (based on the electric double layer capacitance alone),in contrast to the typical EDLC-based specific capacitance of 100-170F/g of prior art graphene-based EDLC supercapacitors.

If at least an electrode contains a redox pair (e.g. graphene and anintrinsically conductive polymer or transition metal oxide), we have aredox or pseudo-capacitor. The supercapacitor is a lithium-ion capacitoror sodium-ion capacitor if the cathode contains the presently inventedelectrode (having graphene or graphene-carbon material mixture as theelectrode active material) and the anode contains a pre-lithiated anodeactive material (e.g. pre-lithiated graphite or Si particles) or apre-sodiated anode active material (e.g. pre-sodiated hard carbonparticles).

The presently invented solid graphene foam may be produced by a processcomprising:

-   (a) preparing a graphene dispersion having a graphene material    dispersed in a liquid medium, wherein the graphene material is    selected from pristine graphene, graphene oxide, reduced graphene    oxide, graphene fluoride, graphene chloride, graphene bromide,    graphene iodide, hydrogenated graphene, nitrogenated graphene,    chemically functionalized graphene, doped graphene, or a combination    thereof and wherein the dispersion contains an optional blowing    agent;-   (b) dispensing and depositing the graphene dispersion onto a surface    of a supporting substrate (e.g. plastic film, rubber sheet, metal    foil, glass sheet, paper sheet, etc.) to form a wet layer of    graphene material, wherein the dispensing and depositing procedure    includes subjecting the graphene dispersion to an    orientation-inducing stress;-   (c) partially or completely removing the liquid medium from the wet    layer of graphene material to form a dried layer of graphene    material having a content of non-carbon elements (e.g. O, H, N, B,    F, Cl, Br, I, etc.) no less than 5% by weight;-   (d) heat treating the dried layer of graphene material at a first    heat treatment temperature from 100° C. to 3,200° C. at a desired    heating rate sufficient to induce volatile gas molecules from the    non-carbon elements or to activate said blowing agent for producing    the solid graphene foam having a density from 0.01 to 1.7 g/cm³    (more typically from 0.1 to 1.7 g/cm³, and even more typically from    0.3 to 1.5 g/cm³, and most typically from 0.5 to 0.1.3 g/cm³), or a    specific surface area from 50 to 3,200 m²/g (more typically from 200    to 2,800 m²/g, and most typically from 500 to 2,500 m²/g); and-   (e) impregnating said multiple pores with a liquid electrolyte or    gel electrolyte to form a layer of pre-impregnated solid graphene    foam, which is subjected to a step of compressing or roll-pressing    that reduces a pore size, improve orientation of graphene planes,    and squeeze excess electrolyte out of the pre-impregnated solid    graphene foam for forming the supercapacitor electrode.

The process may further include a step of heat-treating the solidgraphene foam at a second heat treatment temperature higher than thefirst heat treatment temperature, prior to step (e), for a length oftime sufficient for obtaining a graphene foam wherein the pore wallscontain stacked graphene planes having an inter-plane spacing d₀₀₂ from0.3354 nm to 0.36 nm and a content of non-carbon elements less than 2%by weight.

This optional blowing agent is not required if the graphene material hasa content of non-carbon elements (e.g. O, H, N, B, F, Cl, Br, I, etc.)no less than 5% by weight (preferably no less than 10%, furtherpreferably no less than 20%, even more preferably no less than 30% or40%, and most preferably up to 50%). The subsequent high temperaturetreatment serves to remove a majority of these non-carbon elements fromthe graphene material, generating volatile gas species that producepores or cells in the solid graphene material structure. In other words,quite surprisingly, these non-carbon elements play the role of a blowingagent. Hence, an externally added blowing agent is optional (notrequired). However, the use of a blowing agent can provide addedflexibility in regulating or adjusting the porosity level and pore sizesfor a desired application. The blowing agent is typically required ifthe non-carbon element content is less than 5%, such as pristinegraphene that is essentially all-carbon.

The blowing agent can be a physical blowing agent, a chemical blowingagent, a mixture thereof, a dissolution-and-leaching agent, or amechanically introduced blowing agent.

The process may further include a step of heat-treating the solidgraphene foam at a second heat treatment temperature higher than thefirst heat treatment temperature for a length of time sufficient forobtaining a graphene foam wherein the pore walls contain stackedgraphene planes having an inter-plane spacing d₀₀₂ from 0.3354 nm to0.40 nm and a content of non-carbon elements less than 5% by weight(typically from 0.001% to 2%). When the resulting non-carbon elementcontent is from 0.1% to 2.0%, the inter-plane spacing d₀₀₂ is typicallyfrom 0.337 nm to 0.40 nm.

If the original graphene material in the dispersion contains anon-carbon element content higher than 5% by weight, the graphenematerial in the solid graphene foam (after the heat treatment) containsstructural defects that are induced during the step (d) of heattreating. The liquid medium can be simply water and/or an alcohol, whichis environmentally benign.

In a preferred embodiment, the process is a roll-to-roll process whereinsteps (b) and (c) include feeding the supporting substrate from a feederroller to a deposition zone, continuously or intermittently depositingthe graphene dispersion onto a surface of the supporting substrate toform the wet layer of graphene material thereon, drying the wet layer ofgraphene material to form the dried layer of graphene material, andcollecting the dried layer of graphene material deposited on thesupporting substrate on a collector roller. Such a roll-to-roll orreel-to-reel process is a truly industrial-scale, massive manufacturingprocess that can be automated.

In one embodiment, the first heat treatment temperature is from 100° C.to 1,500° C. In another embodiment, the second heat treatmenttemperature includes at least a temperature selected from (A) 300-1,500°C., (B) 1,500-2,500° C., and/or (C) 2,500-3,200° C. In a specificembodiment, the second heat treatment temperature includes a temperaturein the range of 300-1,500° C. for at least 1 hour and then a temperaturein the range of 1,500-3,200° C. for at least 1 hour.

There are several surprising results of conducting first and/or secondheat treatments to the dried graphene layer, and different heattreatment temperature ranges enable us to achieve different purposes,such as (a) removal of non-carbon elements from the graphene material(e.g. thermal reduction of fluorinated graphene to obtain graphene orreduced graphene fluoride, RGF)) which generate volatile gases toproduce pores or cells in a graphene material, (b) activation of thechemical or physical blowing agent to produce pores or cells, (c)chemical merging or linking of graphene sheets to significantly increasethe lateral dimension of graphene sheets in the foam walls (solidportion of the foam), (d) healing of defects created duringfluorination, oxidation, or nitrogenation of graphene planes in agraphite particle, and (e) re-organization and perfection of graphiticdomains or graphite crystals. These different purposes or functions areachieved to different extents within different temperature ranges. Thenon-carbon elements typically include an element selected from oxygen,fluorine, chlorine, bromine, iodine, nitrogen, hydrogen, or boron. Quitesurprisingly, even under low-temperature foaming conditions,heat-treating induces chemical linking, merging, or chemical bondingbetween graphene sheets, often in an edge-to-edge manner (some inface-to-face manner).

In one embodiment, the sheet of solid graphene foam has a specificsurface area from 200 to 2,500 m²/g. In one embodiment, the sheet ofsolid graphene foam has a density from 0.1 to 1.5 g/cm³. In anembodiment, step (d) of heat treating the layer of graphene material ata first heat treatment temperature is conducted under a compressivestress. In another embodiment, the process comprises a compression stepto reduce a thickness, pore size, or porosity level of the sheet ofgraphene foam. In some applications, the graphene foam has a thicknessno greater than 200 μm.

In an embodiment, the graphene dispersion has at least 3% by weight ofgraphene oxide dispersed in the liquid medium to form a liquid crystalphase. In another embodiment, the graphene dispersion contains agraphene oxide dispersion prepared by immersing a graphitic material ina powder or fibrous form in an oxidizing liquid in a reaction vessel ata reaction temperature for a length of time sufficient to obtain thegraphene dispersion wherein the graphitic material is selected fromnatural graphite, artificial graphite, meso-phase carbon, meso-phasepitch, meso-carbon micro-bead, soft carbon, hard carbon, coke, carbonfiber, carbon nano-fiber, carbon nano-tube, or a combination thereof andwherein the graphene oxide has an oxygen content no less than 5% byweight.

In an embodiment, the first heat treatment temperature contains atemperature in the range of 80° C.-300° C. and, as a result, thegraphene foam has an oxygen content or non-carbon element content lessthan 5%, and the pore walls have an inter-graphene spacing less than0.40 nm, a thermal conductivity of at least 150 W/mK (more typically atleast 200 W/mk) per unit of specific gravity, and/or an electricalconductivity no less than 2,000 S/cm per unit of specific gravity.

In a preferred embodiment, the first and/or second heat treatmenttemperature contains a temperature in the range of 300° C.-1,500° C.and, as a result, the graphene foam has an oxygen content or non-carboncontent less than 1%, and the pore walls have an inter-graphene spacingless than 0.35 nm, a thermal conductivity of at least 250 W/mK per unitof specific gravity, and/or an electrical conductivity no less than2,500 S/cm per unit of specific gravity. When the first and/or secondheat treatment temperature contains a temperature in the range of 1,500°C.-2,100° C., the graphene foam has an oxygen content or non-carboncontent less than 0.01% and pore walls have an inter-graphene spacingless than 0.34 nm, a thermal conductivity of at least 300 W/mK per unitof specific gravity, and/or an electrical conductivity no less than3,000 S/cm per unit of specific gravity.

When the first and/or second heat treatment temperature contains atemperature greater than 2,100° C., the graphene foam has an oxygencontent or non-carbon content no greater than 0.001% and pore walls havean inter-graphene spacing less than 0.336 nm, a mosaic spread value nogreater than 0.7, a thermal conductivity of at least 350 W/mK per unitof specific gravity, and/or an electrical conductivity no less than3,500 S/cm per unit of specific gravity. If the first and/or second heattreatment temperature contains a temperature no less than 2,500° C., thegraphene foam has pore walls containing stacked graphene planes havingan inter-graphene spacing less than 0.336 nm, a mosaic spread value nogreater than 0.4, and a thermal conductivity greater than 400 W/mK perunit of specific gravity, and/or an electrical conductivity greater than4,000 S/cm per unit of specific gravity.

In one embodiment, the pore walls contain stacked graphene planes havingan inter-graphene spacing less than 0.337 nm and a mosaic spread valueless than 1.0. In another embodiment, the solid wall portion of thegraphene foam exhibits a degree of graphitization no less than 80%and/or a mosaic spread value less than 0.4. In yet another embodiment,the solid wall portion of the graphene foam exhibits a degree ofgraphitization no less than 90% and/or a mosaic spread value no greaterthan 0.4.

Typically, the pore walls contain a 3D network of interconnectedgraphene planes that are electron-conducting pathways. The cell wallscontain graphitic domains or graphite crystals having a lateraldimension (L_(a), length or width) no less than 20 nm, more typicallyand preferably no less than 40 nm, still more typically and preferablyno less than 100 nm, still more typically and preferably no less than500 nm, often greater than 1 μm, and sometimes greater than 10 μm. Thegraphitic domains typically have a thickness from 1 nm to 200 nm, moretypically from 1 nm to 100 nm, further more typically from 1 nm to 40nm, and most typically from 1 nm to 30 nm.

Preferably, the solid graphene foam contains pores having a size from0.5 nm to 50 nm. It may be noted that it has not been possible to useNi-catalyzed CVD to produce graphene foams having a pore size range of0.5-50 nm. This is due to the notion that it has not been provenpossible to prepare Ni foam templates having such a pore size range andnot possible for the hydrocarbon gas (precursor molecules) to readilyenter Ni foam pores of these sizes. These Ni foam pores must also beinterconnected. Additionally, the sacrificial plastic colloidal particleapproaches have resulted in macro-pores that are in the size range ofmicrons to millimeters.

In a preferred embodiment, the present invention provides a roll-to-rollprocess for producing a solid graphene foam composed of multiple poresand pore walls The process comprises: (a) preparing a graphenedispersion having a graphene material dispersed in a liquid medium,wherein the dispersion optionally contains a blowing agent; (b)continuously or intermittently dispensing and depositing the graphenedispersion onto a surface of a supporting substrate to form a continuouswet layer of graphene material under an orientation-inducing stress,wherein the supporting substrate is a continuous thin film supplied froma feeder roller and collected on a collector roller; (c) partially orcompletely removing the liquid medium from the wet layer of graphenematerial to form a continuous dried layer of graphene; (d) heat treatingthe dried layer of graphene material at a first heat treatmenttemperature from 100° C. to 1,500° C. at a desired heating ratesufficient to activate the blowing agent for producing a continuouslayer of the solid graphene foam having a density from 0.01 to 1.7 g/cm³or a specific surface area from 50 to 3,200 m²/g; (e) impregnating themultiple pores with the liquid or gel electrolyte to form a continuouslayer of pre-impregnated solid graphene foam, which is subjected to astep of compressing or roll-pressing that reduces a pore size, improveorientation of graphene planes, and squeeze excess electrolyte out ofsaid pre-impregnated solid graphene foam for forming a continuous layerof the supercapacitor electrode; and (f) collecting the continuous layerof supercapacitor electrode on the collector roller. The electrode layermay be subjected to further heat treatments at a second temperature,higher than the first temperature, prior to electrolyte impregnation.The continuous or large-length layer of electrolyte-impregnated foam maybe cut into smaller pieces and multiple pieces may be stacked to form athicker electrode in a supercapacitor.

The orientation-inducing stress may be a shear stress. As an example,the shear stress can be encountered in a situation such as slot-diecoating or reverse roll transfer coating. As another example, aneffective orientation-inducing stress is created in an automatedroll-to-roll coating process in which a “knife-on-roll” configurationdispenses the graphene dispersion over a moving solid substrate, such asa plastic film. The relative motion between this moving film and thecoating knife acts to effect orientation of graphene sheets along theshear stress direction.

This orientation-inducing stress is a critically important step in theproduction of the presently invented graphene foams due to thesurprising observation that the shear stress enables the graphene sheetsto align along a particular direction (e.g. X-direction orlength-direction) to produce preferred orientations and facilitatecontacts between graphene sheets along foam walls. Further surprisingly,these preferred orientations and improved graphene-to-graphene contactsfacilitate chemical merging or linking between graphene sheets duringthe subsequent heat treatment of the dried graphene layer. Suchpreferred orientations and improved contacts are essential to theeventual attainment of exceptionally high thermal conductivity,electrical conductivity, elastic modulus, and mechanical strength of theresulting graphene foam. In general, these great properties could not beobtained without such a shear stress-induced orientation control.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) A flow chart illustrating various prior art processes ofproducing exfoliated graphite products (flexible graphite foils andexpanded graphite flakes), along with a process for producing pristinegraphene foam 40 a or graphene oxide foams 40 b;

FIG. 1(B) Schematic drawing illustrating the processes for producingconventional paper, mat, film, and membrane of simply aggregatedgraphite or NGP flakes/platelets. All processes begin with intercalationand/or oxidation treatment of graphitic materials (e.g. natural graphiteparticles).

FIG. 2 Schematic of a conventional supercapacitor cell.

FIG. 3 A possible mechanism of chemical linking between graphene oxidesheets that effectively increases the graphene sheet lateral dimensions.

FIG. 4(A) Thermal conductivity values vs. specific gravity of the GOsuspension-derived foam produced by the presently invented process,meso-phase pitch-derived graphite foam, and Ni foam-template assistedCVD graphene foam;

FIG. 4(B) Thermal conductivity values of the GO suspension-derived foam,sacrificial plastic bead-templated GO foam, and the hydrothermallyreduced GO graphene foam; and

FIG. 4(C) electrical conductivity data for the GO suspension-derivedfoam produced by the presently invented process and the hydrothermallyreduced GO graphene foam.

FIG. 5(A) Thermal conductivity values (vs. specific gravity values up to1.02 g/cm³) of the GO suspension-derived foam, meso-phase pitch-derivedgraphite foam, and Ni foam-template assisted CVD graphene foam;

FIG. 5(B) Thermal conductivity values of the GO suspension-derived foam,sacrificial plastic bead-templated GO foam, and hydrothermally reducedGO graphene foam (vs. specific gravity values up to 1.02 g/cm³).

FIG. 6 Thermal conductivity values of graphene foam samples derived fromGO and GF (graphene fluoride) as a function of the specific gravity.

FIG. 7 Thermal conductivity values of graphene foam samples derived fromGO and pristine graphene as a function of the final (maximum) heattreatment temperature.

FIG. 8(A) Inter-graphene plane spacing in graphene foam walls asmeasured by X-ray diffraction;

FIG. 8(B) The oxygen content in the GO suspension-derived graphene foam.

FIG. 9 The electrode specific capacitance values of two series ofsupercapacitors (conventional and presently invented) plotted as afunction of the electrode density.

FIG. 10 Ragone plots (gravimetric and volumetric power density vs.energy density) of symmetric supercapacitor (EDLC) cells containingisolated nitrogen-doped graphene sheets or graphene foam as theelectrode active material and EMIMBF4 ionic liquid electrolyte.Supercapacitors were prepared according to an embodiment of instantinvention and, for comparison, by the conventional slurry coating ofelectrodes.

FIG. 11 Ragone plots of lithium ion capacitor (LIC) cells containingpristine graphene sheets as the electrode active material and lithiumsalt-PC/DEC organic liquid electrolyte. Supercapacitors were preparedaccording to an embodiment of instant invention and by the conventionalslurry coating of electrodes.

FIG. 12 The cell-level gravimetric and volumetric energy densitiesplotted over the achievable electrode thickness range of the RGO-basedEDLC supercapacitors (organic liquid electrolyte) prepared via theconventional method and the presently invented method. Legends: thegravimetric (♦) and volumetric (▴) energy density of the conventionalsupercapacitors (highest achieved electrode tap density of approximately0.28 g/cm³) and the gravimetric (▪) and volumetric (x) energy density ofthe inventive supercapacitors (easily achieved electrode tap density ofapproximately 0.8 g/cm³).

FIG. 13 The cell-level gravimetric energy densities plotted over theachievable active material proportion (active material weight/total cellweight) in a supercapacitor cell.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As schematically illustrated in FIG. 2, a prior art supercapacitor cellis typically composed of an anode current collector 202 (e.g. Al foil12-15 μm thick), an anode active material layer 204 (containing an anodeactive material, such as activated carbon particles 232 and conductiveadditives that are bonded by a resin binder, such as PVDF), a porousseparator 230, a cathode active material layer 208 (containing a cathodeactive material, such as activated carbon particles 234, and conductiveadditives that are all bonded by a resin binder, not shown), a cathodecurrent collector 206 (e.g. Al foil), and a liquid electrolyte disposedin both the anode active material layer 204 (also simply referred to asthe “anode layer”) and the cathode active material layer 208 (or simply“cathode layer”). The entire cell is encased in a protective housing,such as a thin plastic-aluminum foil laminate-based envelop. The priorart supercapacitor cell is typically made by a process that includes thefollowing steps:

-   -   a) The first step is mixing particles of the anode active        material (e.g. activated carbon), a conductive filler (e.g.        graphite flakes), a resin binder (e.g. PVDF) in a solvent (e.g.        NMP) to form an anode slurry. On a separate basis, particles of        the cathode active material (e.g. activated carbon), a        conductive filler (e.g. acetylene black), a resin binder (e.g.        PVDF) are mixed and dispersed in a solvent (e.g. NMP) to form a        cathode slurry.    -   b) The second step includes coating the anode slurry onto one or        both primary surfaces of an anode current collector (e.g. Cu or        Al foil), drying the coated layer by vaporizing the solvent        (e.g. NMP) to form a dried anode electrode coated on Cu or Al        foil. Similarly, the cathode slurry is coated and dried to form        a dried cathode electrode coated on Al foil.    -   c) The third step includes laminating an anode/Al foil sheet, a        porous separator layer, and a cathode/Al foil sheet together to        form a 3-layer or 5-layer assembly, which is cut and slit into        desired sizes and stacked to form a rectangular structure (as an        example of shape) or rolled into a cylindrical cell structure.    -   d) The rectangular or cylindrical laminated structure is then        encased in a laminated aluminum-plastic envelope or steel        casing.    -   e) A liquid electrolyte is then injected into the laminated        housing structure to make a supercapacitor cell.

There are several serious problems associated with this conventionalprocess and the resulting supercapacitor cell:

-   -   1) It is very difficult to produce an electrode layer (anode        layer or cathode layer) that is thicker than 100 μm and        practically impossible or impractical to produce an electrode        layer thicker than 200 μm. There are several reasons why this is        the case. An electrode of 100 μm thickness typically requires a        heating zone of 30-50 meters long in a slurry coating facility,        which is too time consuming, too energy intensive, and not        cost-effective. A heating zone longer than 100 meters is not        unusual.    -   2) For some electrode active materials, such as graphene sheets,        it has not been possible to produce an electrode thicker than 50        μm in a real manufacturing environment on a continuous basis.        This is despite the notion that some thicker electrodes have        been claimed in open or patent literature, which were prepared        in a laboratory on a small scale. In a laboratory setting,        presumably one could repeatedly add new materials to a layer and        manually consolidate the layer to increase the thickness of an        electrode. However, even with such a procedure, the resulting        electrode becomes very fragile and brittle. This is even worse        for graphene-based electrodes, since repeated compressions lead        to re-stacking of graphene sheets and, hence, significantly        reduced specific surface area and reduced specific capacitance.    -   3) With a conventional process, as depicted in FIG. 2, the        actual mass loadings of the electrodes and the apparent        densities for the active materials are too low. In most cases,        the active material mass loadings of the electrodes (areal        density) is significantly lower than 10 mg/cm² and the apparent        volume density or tap density of the active material is        typically less than 0.75 g/cm³ (more typically less than 0.5        g/cm³ and most typically less than 0.3 g/cm³) even for        relatively large particles of activated carbon. In addition,        there are so many other non-active materials (e.g. conductive        additive and resin binder) that add additional weights and        volumes to the electrode without contributing to the cell        capacity. These low areal densities and low volume densities        result in relatively low volumetric capacitances and low        volumetric energy density.    -   4) The conventional process requires dispersing electrode active        materials (anode active material and cathode active material) in        a liquid solvent (e.g. NMP) to make a wet slurry and, upon        coating on a current collector surface, the liquid solvent has        to be removed to dry the electrode layer. Once the anode and        cathode layers, along with a separator layer, are laminated        together and packaged in a housing to make a supercapacitor        cell, one then injects a liquid electrolyte into the cell. In        actuality, one makes the two electrodes wet, then makes the        electrodes dry, and finally makes them wet again. Such a        wet-dry-wet process is clearly not a good process at all.    -   5) Current supercapacitors (e.g. symmetric supercapacitors or        electric double layer capacitors, EDLC) still suffer from a        relatively low gravimetric energy density and low volumetric        energy density. Commercially available EDLCs exhibit a        gravimetric energy density of approximately 6 Wh/kg and no        experimental EDLC cells have been reported to exhibit an energy        density higher than 10 Wh/kg (based on the total cell weight) at        room temperature. Although experimental supercapacitors exhibit        large volumetric electrode capacitances (100 to 200 F/cm³ in        most cases) at the electrode level (not the cell level), their        typical active mass loading of <1 mg/cm², tap density of <0.1        g/cm³, and electrode thicknesses of up to tens of micrometers        remain significantly lower than those used in most commercially        available electrochemical capacitors, resulting in energy        storage devices with relatively low areal and volumetric        capacities and low volumetric energy densities based on the cell        (device) weight.        -   In literature, the energy density data reported based on            either the active material weight alone or electrode weight            cannot directly translate into the energy densities of a            practical supercapacitor cell or device. The “overhead            weight” or weights of other device components (binder,            conductive additive, current collectors, separator,            electrolyte, and packaging) must also be taken into account.            The convention production process results in an active            material proportion being less than 30% by weight of the            total cell weight (<15% in some cases; e.g. for            graphene-based active material).

The present invention provides a process for producing a supercapacitorcell having a high electrode thickness (no theoretical limitation on theelectrode thickness that can be made by using the present process), highactive material mass loading, low overhead weight and volume, highvolumetric capacitance, and high volumetric energy density. Theelectrode produced can be directly impregnated with an electrolyte(aqueous, organic, ionic liquid, or polymer gel) without going throughthe lengthy and environmentally unfriendly wet-dry-wet procedures of theprior art process.

In some preferred embodiments, the present invention provides asupercapacitor electrode comprising a solid graphene foam impregnatedwith a liquid or gel electrolyte, wherein the solid graphene foam iscomposed of multiple pores and pore walls, wherein the pore wallscontain a pristine graphene material having essentially zero % ofnon-carbon elements, or a non-pristine graphene material having 0.001%to 5% by weight of non-carbon elements wherein the non-pristine grapheneis selected from graphene oxide, reduced graphene oxide, graphenefluoride, graphene chloride, graphene bromide, graphene iodide,hydrogenated graphene, nitrogenated graphene, doped graphene, chemicallyfunctionalized graphene, doped graphene, or a combination thereof, andthe solid graphene foam, when measured in a dried state without saidelectrolyte, has a physical density from 0.01 to 1.7 g/cm³, a specificsurface area from 50 to 3,300 m²/g, a thermal conductivity of at least200 W/mK per unit of specific gravity, and/or an electrical conductivityno less than 2,000 S/cm per unit of specific gravity. The pore wallspreferably and typically contain stacked graphene planes having aninter-plane spacing d₀₀₂ from 0.3354 nm to 0.40 nm as measured by X-raydiffraction.

In certain preferred embodiments, the present invention provides a sheetof solid graphene foam composed of multiple pores and pore walls, whichare then impregnated with an electrolyte. The pores in the graphene foamare formed slightly before, during, or after sheets of a graphenematerial are (1) chemically linked/merged together (edge-to-edge and/orface-to-face) typically at a temperature from 100 to 1,500° C. and/or(2) re-organized into larger graphite crystals or domains (hereinreferred to as re-graphitization) along the pore walls at a hightemperature (typically >2,100° C. and more typically >2,500° C.).Optionally, the electrolyte-impregnated graphene foam is then compressedor roll-pressed to improve the wetting of graphene walls by the liquidor gel electrolyte, squeeze out excess electrolyte, and reduce the poresizes or separation between graphene sheets for enhanced tap density.

In a preferred embodiment, the process comprises:

(a) preparing a graphene dispersion having a graphene material dispersedin a liquid medium, wherein the graphene material is selected frompristine graphene, graphene oxide, reduced graphene oxide, graphenefluoride, graphene chloride, graphene bromide, graphene iodide,hydrogenated graphene, nitrogenated graphene, chemically functionalizedgraphene, doped graphene, or a combination thereof and wherein thedispersion contains an optional blowing agent with a blowingagent-to-graphene material weight ratio from 0/1.0 to 1.0/1.0 (thisblowing agent is normally required if the graphene material is pristinegraphene, typically having a blowing agent-to-pristine graphene weightratio from 0.01/1.0 to 1.0/1.0);

(b) dispensing and depositing the graphene dispersion onto a surface ofa supporting substrate (e.g. plastic film, rubber sheet, metal foil,glass sheet, paper sheet, etc.) to form a first wet layer of graphenematerial, wherein the dispensing and depositing procedure (e.g. coatingor casting) includes subjecting the graphene dispersion to anorientation-inducing stress (preferably entailing a shear stress);

(c) partially or completely removing the liquid medium from the firstwet layer of graphene material to form a first dried layer of graphenematerial having a content of non-carbon elements (e.g. O, H, N, B, F,Cl, Br, I, etc.) no less than 5% by weight (this non-carbon content,when being removed via heat-induced decomposition, produces volatilegases that act as a foaming agent or blowing agent);

(d) heat treating the first layer of graphene material at a first heattreatment temperature from 100° C. to 3,000° C. at a desired heatingrate sufficient to induce volatile gas molecules from the non-carbonelements or to activate said blowing agent for producing the solidgraphene foam. The graphene foam typically has a density from 0.01 to1.7 g/cm³ (more typically from 0.1 to 1.5 g/cm³, and even more typicallyfrom 0.3 to 1.3 g/cm³, and most typically from 0.5 to 1.1 g/cm³), or aspecific surface area from 50 to 3,200 m²/g (more typically from 200 to2,800 m²/g, and most typically from 500 to 2,500 m²/g). Optionally, thespecific surface area can be further increased by subjecting thegraphene foam to a chemical or physical activation treatment (e.g.mixing with KOH at 700-900° C. for 2-6 hours).

(e) impregnating the pores of graphene foam with a liquid or gelelectrolyte to form electrolyte-impregnated graphene foam, which is thenoptionally compressed (e.g. roll-pressed) to increase the tap density ofthe foam.

A blowing agent or foaming agent is a substance which is capable ofproducing a cellular or foamed structure via a foaming process in avariety of materials that undergo hardening or phase transition, such aspolymers (plastics and rubbers), glass, and metals. They are typicallyapplied when the material being foamed is in a liquid state. It has notbeen previously known that a blowing agent can be used to create afoamed material while in a solid state. More significantly, it has notbeen taught or hinted that an aggregate of sheets of a graphene materialcan be converted into a graphene foam via a blowing agent. The cellularstructure in a matrix is typically created for the purpose of reducingdensity, increasing thermal resistance and acoustic insulation, whileincreasing the thickness and relative stiffness of the original polymer.

Blowing agents or related foaming mechanisms to create pores or cells(bubbles) in a matrix for producing a foamed or cellular material, canbe classified into the following groups:

-   -   (a) Physical blowing agents: e.g. hydrocarbons (e.g. pentane,        isopentane, cyclopentane), chlorofluorocarbons (CFCs),        hydrochlorofluorocarbons (HCFCs), and liquid CO₂. The        bubble/foam-producing process is endothermic, i.e. it needs heat        (e.g. from a melt process or the chemical exotherm due to        cross-linking), to volatize a liquid blowing agent.    -   (b) Chemical blowing agents: e.g. isocyanate, azo-, hydrazine        and other nitrogen-based materials (for thermoplastic and        elastomeric foams), sodium bicarbonate (e.g. baking soda, used        in thermoplastic foams). Here gaseous products and other        by-products are formed by a chemical reaction, promoted by        process or a reacting polymer's exothermic heat. Since the        blowing reaction involves forming low molecular weight compounds        that act as the blowing gas, additional exothermic heat is also        released. Powdered titanium hydride is used as a foaming agent        in the production of metal foams, as it decomposes to form        titanium and hydrogen gas at elevated temperatures.        Zirconium (II) hydride is used for the same purpose. Once formed        the low molecular weight compounds will never revert to the        original blowing agent(s), i.e. the reaction is irreversible.    -   (c) Mixed physical/chemical blowing agents: e.g. used to produce        flexible polyurethane (PU) foams with very low densities. Both        the chemical and physical blowing can be used in tandem to        balance each other out with respect to thermal energy        released/absorbed; hence, minimizing temperature rise. For        instance, isocyanate and water (which react to form CO₂) are        used in combination with liquid CO₂ (which boils to give gaseous        form) in the production of very low density flexible PU foams        for mattresses.    -   (d) Mechanically injected agents: Mechanically made foams        involve methods of introducing bubbles into liquid polymerizable        matrices (e.g. an unvulcanized elastomer in the form of a liquid        latex). Methods include whisking-in air or other gases or low        boiling volatile liquids in low viscosity lattices, or the        injection of a gas into an extruder barrel or a die, or into        injection molding barrels or nozzles and allowing the shear/mix        action of the screw to disperse the gas uniformly to form very        fine bubbles or a solution of gas in the melt. When the melt is        molded or extruded and the part is at atmospheric pressure, the        gas comes out of solution expanding the polymer melt immediately        before solidification.    -   (e) Soluble and leachable agents: Soluble fillers, e.g. solid        sodium chloride crystals mixed into a liquid urethane system,        which is then shaped into a solid polymer part, the sodium        chloride is later washed out by immersing the solid molded part        in water for some time, to leave small inter-connected holes in        relatively high density polymer products.    -   (f) We have found that the above five mechanisms can all be used        to create pores in the graphene materials while they are in a        solid state. Another mechanism of producing pores in a graphene        material is through the generation and vaporization of volatile        gases by removing those non-carbon elements in a        high-temperature environment. This is a unique self-foaming        process that has never been previously taught or suggested.

In a preferred embodiment, the graphene material in the dispersion isselected from pristine graphene, graphene oxide, reduced graphene oxide,graphene fluoride, graphene chloride, graphene bromide, graphene iodide,hydrogenated graphene, nitrogenated graphene, chemically functionalizedgraphene, doped graphene, or a combination thereof. The startinggraphitic material for producing any one of the above graphene materialsmay be selected from natural graphite, artificial graphite, meso-phasecarbon, meso-phase pitch, meso-carbon micro-bead, soft carbon, hardcarbon, coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or acombination thereof.

For instance, the graphene oxide (GO) may be obtained by immersingpowders or filaments of a starting graphitic material (e.g. naturalgraphite powder) in an oxidizing liquid medium (e.g. a mixture ofsulfuric acid, nitric acid, and potassium permanganate) in a reactionvessel at a desired temperature for a period of time (typically from 0.5to 96 hours, depending upon the nature of the starting material and thetype of oxidizing agent used). The resulting graphite oxide particlesmay then be subjected to thermal exfoliation or ultrasonic wave-inducedexfoliation to produce GO sheets.

Pristine graphene may be produced by direct ultrasonication (also knownas liquid phase production) or supercritical fluid exfoliation ofgraphite particles. These processes are well-known in the art. Multiplepristine graphene sheets may be dispersed in water or other liquidmedium with the assistance of a surfactant to form a suspension. Achemical blowing agent may then be dispersed into the dispersion (38 inFIG. 1(A)). This suspension is then cast or coated onto the surface of asolid substrate (e.g. glass sheet or Al foil). When heated to a desiredtemperature, the chemical blowing agent is activated or decomposed togenerate volatile gases (e.g. N₂ or CO₂), which act to form bubbles orpores in an otherwise mass of solid graphene sheets, forming a pristinegraphene foam 40 a.

Fluorinated graphene or graphene fluoride is herein used as an exampleof the halogenated graphene material group. There are two differentapproaches that have been followed to produce fluorinated graphene: (1)fluorination of pre-synthesized graphene: This approach entails treatinggraphene prepared by mechanical exfoliation or by CVD growth withfluorinating agent such as XeF₂, or F-based plasmas; (2) Exfoliation ofmultilayered graphite fluorides: Both mechanical exfoliation and liquidphase exfoliation of graphite fluoride can be readily accomplished [F.Karlicky, et al. “Halogenated Graphenes: Rapidly Growing Family ofGraphene Derivatives” ACS Nano, 2013, 7 (8), pp 6434-6464].

Interaction of F₂ with graphite at high temperature leads to covalentgraphite fluorides (CF)_(n) or (C₂F)_(n), while at low temperaturesgraphite intercalation compounds (GIC) C_(x)F (2≤x≤24) form. In (CF)_(n)carbon atoms are sp3-hybridized and thus the fluorocarbon layers arecorrugated consisting of trans-linked cyclohexane chairs. In (C₂F)_(n)only half of the C atoms are fluorinated and every pair of the adjacentcarbon sheets are linked together by covalent C—C bonds. Systematicstudies on the fluorination reaction showed that the resulting F/C ratiois largely dependent on the fluorination temperature, the partialpressure of the fluorine in the fluorinating gas, and physicalcharacteristics of the graphite precursor, including the degree ofgraphitization, particle size, and specific surface area. In addition tofluorine (F₂), other fluorinating agents may be used, although most ofthe available literature involves fluorination with F₂ gas, sometimes inpresence of fluorides.

For exfoliating a layered precursor material to the state of individuallayers or few-layers, it is necessary to overcome the attractive forcesbetween adjacent layers and to further stabilize the layers. This may beachieved by either covalent modification of the graphene surface byfunctional groups or by non-covalent modification using specificsolvents, surfactants, polymers, or donor-acceptor aromatic molecules.The process of liquid phase exfoliation includes ultrasonic treatment ofa graphite fluoride in a liquid medium.

The nitrogenation of graphene can be conducted by exposing a graphenematerial, such as graphene oxide, to ammonia at high temperatures(200-400° C.). Nitrogenated graphene could also be formed at lowertemperatures by a hydrothermal method; e.g. by sealing GO and ammonia inan autoclave and then increased the temperature to 150-250° C. Othermethods to synthesize nitrogen doped graphene include nitrogen plasmatreatment on graphene, arc-discharge between graphite electrodes in thepresence of ammonia, ammonolysis of graphene oxide under CVD conditions,and hydrothermal treatment of graphene oxide and urea at differenttemperatures.

The pore walls (cell walls) in the presently invented graphene foamcontain chemically bonded and merged graphene planes. These planararomatic molecules or graphene planes (hexagonal structured carbonatoms) are well interconnected physically and chemically. The lateraldimensions (length or width) of these planes are huge (from 20 nm to >10μm), typically several times or even orders of magnitude larger than themaximum crystallite dimension (or maximum constituent graphene planedimension) of the starting graphite particles. The graphene sheets orplanes are essentially interconnected to form electron-conductingpathways with low resistance. This is a unique and new class of materialthat has not been previously discovered, developed, or suggested topossibly exist.

In order to illustrate how the presently invented process works toproduce a graphene foam, we herein make use of graphene oxide (GO) andgraphene fluoride (GF) as two examples. These should not be construed aslimiting the scope of our claims. In each case, the first step involvespreparation of a graphene dispersion (e.g. GO+water or GF+organicsolvent, DMF) containing an optional blowing agent. If the graphenematerial is pristine graphene containing no non-carbon elements, ablowing agent is required.

In step (b), the GF or GO suspension (21 in FIG. 1(A)) is formed into awet GF or GO layer 35 on a solid substrate surface (e.g. PET film orglass) preferably under the influence of a shear stress. One example ofsuch a shearing procedure is casting or coating a thin film of GF or GOsuspension using a coating machine. This procedure is similar to a layerof varnish, paint, coating, or ink being coated onto a solid substrate.The roller, “doctor's blade”, or wiper creates a shear stress when thefilm is shaped, or when there is a relative motion between theroller/blade/wiper and the supporting substrate. Quite unexpectedly andsignificantly, such a shearing action enables the planar GF or GO sheetsto well align along, for instance, a shearing direction. Furthersurprisingly, such a molecular alignment state or preferred orientationis not disrupted when the liquid components in the GF or GO suspensionare subsequently removed to form a well-packed layer of highly alignedGF or GO sheets that are at least partially dried. The dried GF or GOmass 37 a has a high birefringence coefficient between an in-planedirection and the normal-to-plane direction.

In an embodiment, this GF or GO layer is then subjected to a heattreatment to activate the blowing agent and/or the thermally-inducedreactions that remove the non-carbon elements (e.g. F, O, etc.) from thegraphene sheets to generate volatile gases as by-products. Thesevolatile gases generate pores or bubbles inside the solid graphenematerial, pushing solid graphene sheets into a wall structure, forming agraphene oxide foam 40 b. If no blowing agent is added, the non-carbonelements in the graphene material preferably occupy at least 10% byweight of the graphene material (preferably at least 20%, and furtherpreferably at least 30%). The first (initial) heat treatment temperatureis typically greater than 80° C., preferably greater than 100° C., morepreferably greater than 300° C., further more preferably greater than500° C. and can be as high as 1,500° C. The blowing agent is typicallyactivated at a temperature from 80° C. to 300° C., but can be higher.The foaming procedure (formation of pores, cells, or bubbles) istypically completed within the temperature range of 80-1,500° C. Quitesurprisingly, the chemical linking or merging between graphene planes(GO or GF planes) in an edge-to-edge and face-to-face manner can occurat a relatively low heat treatment temperature (e.g. as low as from 150to 300° C.).

The foamed graphene material may be subjected to a further heattreatment that involves at least a second temperature that issignificantly higher than the first heat treatment temperature.

A properly programmed heat treatment procedure can involve just a singleheat treatment temperature (e.g. a first heat treatment temperatureonly), at least two heat treatment temperatures (first temperature for aperiod of time and then raised to a second temperature and maintained atthis second temperature for another period of time), or any othercombination of heat treatment temperatures (HTT) that involve an initialtreatment temperature (first temperature) and a final HTT (second),higher than the first. The highest or final HTT that the dried graphenelayer experiences may be divided into four distinct HTT regimes:

-   Regime 1 (80° C. to 300° C.): In this temperature range (the thermal    reduction regime and also the activation regime for a blowing agent,    if present), a GO or GF layer primarily undergoes thermally-induced    reduction reactions, leading to a reduction of oxygen content or    fluorine content from typically 20-50% (of O in GO) or 10-25% (of F    in GF) to approximately 5-6%. This treatment results in a reduction    of inter-graphene spacing in foam walls from approximately 0.6-1.2    nm (as dried) down to approximately 0.4 nm, and an increase in    thermal conductivity to 200 W/mK per unit specific gravity and/or    electrical conductivity to 2,000 S/cm per unit of specific gravity.    (Since one can vary the level of porosity and, hence, specific    gravity of a graphene foam material and, given the same graphene    material, both the thermal conductivity and electric conductivity    values vary with the specific gravity, these property values must be    divided by the specific gravity to facilitate a fair comparison.)    Even with such a low temperature range, some chemical linking    between graphene sheets occurs. The inter-GO or inter-GF planar    spacing remains relatively large (0.4 nm or larger). Many O- or    F-containing functional groups survive.-   Regime 2 (300° C.-1,500° C.): In this chemical linking regime,    extensive chemical combination, polymerization, and cross-linking    between adjacent GO or GF sheets occur. The oxygen or fluorine    content is reduced to typically <1.0% (e.g. 0.7%) after chemical    linking, resulting in a reduction of inter-graphene spacing to    approximately 0.345 nm. This implies that some initial    re-graphitization has already begun at such a low temperature, in    stark contrast to conventional graphitizable materials (such as    carbonized polyimide film) that typically require a temperature as    high as 2,500° C. to initiate graphitization. This is another    distinct feature of the presently invented graphene foam and its    production processes. These chemical linking reactions result in an    increase in thermal conductivity to 250 W/mK per unit of specific    gravity, and/or electrical conductivity to 2,500-4,000 S/cm per unit    of specific gravity.-   Regime 3 (1,500-2,500° C.): In this ordering and re-graphitization    regime, extensive graphitization or graphene plane merging occurs,    leading to significantly improved degree of structural ordering in    the foam walls. As a result, the oxygen or fluorine content is    reduced to typically 0.01% and the inter-graphene spacing to    approximately 0.337 nm (achieving degree of graphitization from 1%    to approximately 80%, depending upon the actual HTT and length of    time). The improved degree of ordering is also reflected by an    increase in thermal conductivity to >350 W/mK per unit of specific    gravity, and/or electrical conductivity to >3,500 S/cm per unit of    specific gravity.-   Regime 4 (higher than 2,500° C.): In this re-crystallization and    perfection regime, extensive movement and elimination of grain    boundaries and other defects occur, resulting in the formation of    nearly perfect single crystals or poly-crystalline graphene crystals    with huge grains in the foam walls, which can be orders of magnitude    larger than the original grain sizes of the starting graphite    particles for the production of GO or GF. The oxygen or fluorine    content is essentially eliminated, typically 0%-0.001%. The    inter-graphene spacing is reduced to down to approximately 0.3354 nm    (degree of graphitization from 80% to nearly 100%), corresponding to    that of a perfect graphite single crystal. The foamed structure thus    obtained exhibits a thermal conductivity of >400 W/mK per unit of    specific gravity, and electrical conductivity of >4,000 S/cm per    unit of specific gravity.

The presently invented graphene foam structure can be obtained byheat-treating the dried GO or GF layer with a temperature program thatcovers at least the first regime (typically requiring 1-4 hours in thistemperature range if the temperature never exceeds 500° C.), morecommonly covers the first two regimes (1-2 hours preferred), still morecommonly the first three regimes (preferably 0.5-2.0 hours in Regime 3),and can cover all the 4 regimes (including Regime 4 for 0.2 to 1 hour,may be implemented to achieve the highest conductivity).

If the graphene material is selected from the group of non-pristinegraphene materials consisting of graphene oxide, reduced graphene oxide,graphene fluoride, graphene chloride, graphene bromide, graphene iodide,hydrogenated graphene, nitrogenated graphene, chemically functionalizedgraphene, doped graphene, or a combination thereof, and wherein themaximum heat treatment temperature (e.g. both the first and second heattreatment temperatures) is (are) less than 2,500° C., then the resultingsolid graphene foam typically contains a content of non-carbon elementsin the range of 0.01% to 2.0% by weight (non-pristine graphene foam).

X-ray diffraction patterns were obtained with an X-ray diffractometerequipped with CuKcv radiation. The shift and broadening of diffractionpeaks were calibrated using a silicon powder standard. The degree ofgraphitization, g, was calculated from the X-ray pattern using theMering's Eq, d₀₀₂=0.3354 g+0.344 (1-g), where d₀₀₂ is the interlayerspacing of graphite or graphene crystal in nm. This equation is validonly when d₀₀₂ is equal or less than approximately 0.3440 nm. Thegraphene foam walls having a d₀₀₂ higher than 0.3440 nm reflects thepresence of oxygen- or fluorine-containing functional groups (such as—F, —OH, >O, and —COOH on graphene molecular plane surfaces or edges)that act as a spacer to increase the inter-graphene spacing.

Another structural index that can be used to characterize the degree ofordering of the stacked and bonded graphene planes in the foam walls ofgraphene and conventional graphite crystals is the “mosaic spread,”which is expressed by the full width at half maximum of a rocking curve(X-ray diffraction intensity) of the (002) or (004) reflection. Thisdegree of ordering characterizes the graphite or graphene crystal size(or grain size), amounts of grain boundaries and other defects, and thedegree of preferred grain orientation. A nearly perfect single crystalof graphite is characterized by having a mosaic spread value of 0.2-0.4.Most of our graphene walls have a mosaic spread value in this range of0.2-0.4 (if produced with a heat treatment temperature (HTT) no lessthan 2,500° C.). However, some values are in the range of 0.4-0.7 if theHTT is between 1,500 and 2,500° C., and in the range of 0.7-1.0 if theHTT is between 300 and 1,500° C.

Illustrated in FIG. 3 is a plausible chemical linking mechanism whereonly 2 aligned GO molecules are shown as an example, although a largenumber of GO molecules can be chemically linked together to form a foamwall. Further, chemical linking could also occur face-to-face, not justedge-to-edge for GO, GF, and chemically functionalized graphene sheets.These linking and merging reactions proceed in such a manner that themolecules are chemically merged, linked, and integrated into one singleentity. The graphene sheets (GO or GF sheets) completely lose their ownoriginal identity and they no longer are discretesheets/platelets/flakes. The resulting product is not a simple aggregateof individual graphene sheets, but a single entity that is essentially anetwork of interconnected giant molecules with an essentially infinitemolecular weight. This may also be described as a graphene poly-crystal(with several grains, but typically no discernible, well-defined grainboundaries). All the constituent graphene planes are very large inlateral dimensions (length and width) and, if the HTT is sufficientlyhigh (e.g. >1,500° C. or much higher), these graphene planes areessentially bonded together with one another.

In-depth studies using a combination of SEM, TEM, selected areadiffraction, X-ray diffraction, AFM, Raman spectroscopy, and FTIRindicate that the graphene foam walls are composed of several hugegraphene planes (with length/width typically >>20 nm, moretypically >>100 nm, often >>1 μm, and, in many cases, >>10 μm, oreven >>100 μm). These giant graphene planes are stacked and bonded alongthe thickness direction (crystallographic c-axis direction) oftenthrough not just the van der Waals forces (as in conventional graphitecrystallites), but also covalent bonds, if the final heat treatmenttemperature is lower than 2,500° C. In these cases, wishing not to belimited by theory, but Raman and FTIR spectroscopy studies appear toindicate the co-existence of sp² (dominating) and sp³ (weak butexisting) electronic configurations, not just the conventional sp² ingraphite.

-   (1) This graphene foam wall is not made by gluing or bonding    discrete flakes/platelets together with a resin binder, linker, or    adhesive. Instead, GO sheets (molecules) from the GO dispersion or    the GF sheets from the GF dispersion are merged through joining or    forming of covalent bonds with one another, into an integrated    graphene entity, without using any externally added linker or binder    molecules or polymers.-   (2) This graphene foam wall is typically a poly-crystal composed of    large grains having incomplete grain boundaries. This entity is    derived from a GO or GF suspension, which is in turn obtained from    natural graphite or artificial graphite particles originally having    multiple graphite crystallites. Prior to being chemically oxidized    or fluorinated, these starting graphite crystallites have an initial    length (L_(a) in the crystallographic a-axis direction), initial    width (L_(b) in the b-axis direction), and thickness (L_(c) in the    c-axis direction). Upon oxidation or fluorination, these initially    discrete graphite particles are chemically transformed into highly    aromatic graphene oxide or graphene fluoride molecules having a    significant concentration of edge- or surface-borne functional    groups (e.g. —F, —OH, —COOH, etc.). These aromatic GO or GF    molecules in the suspension have lost their original identity of    being part of a graphite particle or flake. Upon removal of the    liquid component from the suspension, the resulting GO or GF    molecules form an essentially amorphous structure. Upon heat    treatments, these GO or GF molecules are chemically merged and    linked into a unitary or monolithic graphene entity that constitutes    the foam wall. This foam wall is highly ordered.

The resulting unitary graphene entity in the foam wall typically has alength or width significantly greater than the L_(a) and L_(b) of theoriginal crystallites. The length/width of this graphene foam wallentity is significantly greater than the L_(a) and L_(b) of the originalcrystallites. Even the individual grains in a poly-crystalline graphenewall structure have a length or width significantly greater than theL_(a) and L_(b) of the original crystallites.

-   (3) Due to these unique chemical composition (including oxygen or    fluorine content), morphology, crystal structure (including    inter-graphene spacing), and structural features (e.g. high degree    of orientations, few defects, incomplete grain boundaries, chemical    bonding and no gap between graphene sheets, and substantially no    interruptions in graphene planes), the GO- or GF-derived graphene    foam has a unique combination of outstanding thermal conductivity,    electrical conductivity, mechanical strength, and stiffness (elastic    modulus).

The aforementioned features are further described and explained indetail as follows: As illustrated in FIG. 1(B), a graphite particle(e.g. 100) is typically composed of multiple graphite crystallites orgrains. A graphite crystallite is made up of layer planes of hexagonalnetworks of carbon atoms. These layer planes of hexagonally arrangedcarbon atoms are substantially flat and are oriented or ordered so as tobe substantially parallel and equidistant to one another in a particularcrystallite. These layers of hexagonal-structured carbon atoms, commonlyreferred to as graphene layers or basal planes, are weakly bondedtogether in their thickness direction (crystallographic c-axisdirection) by weak van der Waals forces and groups of these graphenelayers are arranged in crystallites. The graphite crystallite structureis usually characterized in terms of two axes or directions: the c-axisdirection and the a-axis (or b-axis) direction. The c-axis is thedirection perpendicular to the basal planes. The a- or b-axes are thedirections parallel to the basal planes (perpendicular to the c-axisdirection).

A highly ordered graphite particle can consist of crystallites of aconsiderable size, having a length of L_(a) along the crystallographica-axis direction, a width of L_(b) along the crystallographic b-axisdirection, and a thickness L_(c) along the crystallographic c-axisdirection. The constituent graphene planes of a crystallite are highlyaligned or oriented with respect to each other and, hence, theseanisotropic structures give rise to many properties that are highlydirectional. For instance, the thermal and electrical conductivity of acrystallite are of great magnitude along the plane directions (a- orb-axis directions), but relatively low in the perpendicular direction(c-axis). As illustrated in the upper-left portion of FIG. 1(B),different crystallites in a graphite particle are typically oriented indifferent directions and, hence, a particular property of amulti-crystallite graphite particle is the directional average value ofall the constituent crystallites.

Due to the weak van der Waals forces holding the parallel graphenelayers, natural graphite can be treated so that the spacing between thegraphene layers can be appreciably opened up so as to provide a markedexpansion in the c-axis direction, and thus form an expanded graphitestructure in which the laminar character of the carbon layers issubstantially retained. The process for manufacturing flexible graphiteis well-known in the art. In general, flakes of natural graphite (e.g.100 in FIG. 1(B)) are intercalated in an acid solution to producegraphite intercalation compounds (GICs, 102). The GICs are washed,dried, and then exfoliated by exposure to a high temperature for a shortperiod of time. This causes the flakes to expand or exfoliate in thec-axis direction of the graphite up to 80-300 times of their originaldimensions. The exfoliated graphite flakes are vermiform in appearanceand, hence, are commonly referred to as worms 104. These worms ofgraphite flakes which have been greatly expanded can be formed withoutthe use of a binder into cohesive or integrated sheets of expandedgraphite, e.g. webs, papers, strips, tapes, foils, mats or the like(typically referred to as “flexible graphite” 106) having a typicaldensity of about 0.04-2.0 g/cm³ for most applications.

The upper left portion of FIG. 1(A) shows a flow chart that illustratesthe prior art processes used to fabricate flexible graphite foils. Theprocesses typically begin with intercalating graphite particles 20(e.g., natural graphite or synthetic graphite) with an intercalant(typically a strong acid or acid mixture) to obtain a graphiteintercalation compound 22 (GIC). After rinsing in water to remove excessacid, the GIC becomes “expandable graphite.” The GIC or expandablegraphite is then exposed to a high temperature environment (e.g., in atube furnace preset at a temperature in the range of 800-1,050° C.) fora short duration of time (typically from 15 seconds to 2 minutes). Thisthermal treatment allows the graphite to expand in its c-axis directionby a factor of 30 to several hundreds to obtain a worm-like vermicularstructure 24 (graphite worm), which contains exfoliated, butun-separated graphite flakes with large pores interposed between theseinterconnected flakes.

In one prior art process, the exfoliated graphite (or mass of graphiteworms) is re-compressed by using a calendaring or roll-pressingtechnique to obtain flexible graphite foils (26 in FIG. 1(A) or 106 inFIG. 1(B)), which are typically 100-300 μm thick. In another prior artprocess, the exfoliated graphite worm 24 may be impregnated with a resinand then compressed and cured to form a flexible graphite composite,which is normally of low strength as well. In addition, upon resinimpregnation, the electrical and thermal conductivity of the graphiteworms could be reduced by two orders of magnitude.

Alternatively, the exfoliated graphite may be subjected tohigh-intensity mechanical shearing/separation treatments using ahigh-intensity air jet mill, high-intensity ball mill, or ultrasonicdevice to produce separated nano graphene platelets 33 (NGPs) with allthe graphene platelets thinner than 100 nm, mostly thinner than 10 nm,and, in many cases, being single-layer graphene (also illustrated as 112in FIG. 1(B)). An NGP is composed of a graphene sheet or a plurality ofgraphene sheets with each sheet being a two-dimensional, hexagonalstructure of carbon atoms. A mass of multiple NGPs (including discretesheets/platelets of single-layer and/or few-layer graphene or grapheneoxide, 33 in FIG. 1(A)) may be made into a graphene film/paper (34 inFIG. 1(A) or 114 in FIG. 1(B)) using a film- or paper-making process.

Further alternatively, with a low-intensity shearing, graphite wormstend to be separated into the so-called expanded graphite flakes (108 inFIG. 1(B) having a thickness >100 nm. These flakes can be formed intographite paper or mat 106 using a paper- or mat-making process. Thisexpanded graphite paper or mat 106 is just a simple aggregate or stackof discrete flakes having defects, interruptions, and mis-orientationsbetween these discrete flakes.

The solid graphene foam produced by the presently invented process maybe further subjected to the following treatments, separately or incombination:

-   -   (a) Being chemically functionalized or doped with atomic, ionic,        or molecular species. Useful surface functional groups may        include quinone, hydroquinone, quaternized aromatic amines,        mercaptans, or disulfides. This class of functional groups can        impart pseudo-capacitance to graphene-based supercapacitors.    -   (b) coated or grafted with an intrinsically conductive polymer        (conducting polymers, such as polyacetylene, polypyrrole,        polyaniline, polythiophene, and their derivatives, are good        choices for use in the present invention); These treatments are        intended for further increasing the capacitance value through        pseudo-capacitance effects such as redox reactions.    -   (c) deposition with transition metal oxides or sulfides, such as        RuO₂, TiO₂, MnO₂, Cr₂O₃, and Co₂O₃, for the purpose of forming        redox pairs with graphene sheets, thereby imparting        pseudo-capacitance to the electrode; and    -   (d) subjected to an activation treatment (analogous to        activation of carbon black materials) to create additional        surfaces and possibly imparting functional chemical groups to        these surfaces. The activation treatment can be accomplished        through CO₂ physical activation, KOH chemical activation, or        exposure to nitric acid, fluorine, or ammonia plasma.

In the instant invention, there is no limitation on the type of liquidor gel electrolytes that can be used in the supercapacitor: aqueous,organic, gel, and ionic liquid. Typically, electrolytes forsupercapacitors consist of solvent and dissolved chemicals (e.g. salts)that dissociate into positive ions (cations) and negative ions (anions),making the electrolyte electrically conductive. The more ions theelectrolyte contains, the better its conductivity, which also influencesthe capacitance. In supercapacitors, the electrolyte provides themolecules for the separating monolayer in the Helmholtz double-layer(electric double layer) and delivers the ions for pseudocapacitance.

Water is a relatively good solvent for dissolving inorganic chemicals.When added together with acids such as sulfuric acid (H₂SO₄), alkalissuch as potassium hydroxide (KOH), or salts such as quaternaryphosphonium salts, sodium perchlorate (NaClO₄), lithium perchlorate(LiClO₄) or lithium hexafluoride arsenate (LiAsF₆), water offersrelatively high conductivity values. Aqueous electrolytes have adissociation voltage of 1.15 V per electrode and a relatively lowoperating temperature range. Water electrolyte-based supercapacitorsexhibit low energy density.

Alternatively, electrolytes may contain organic solvents, such asacetonitrile, propylene carbonate, tetrahydrofuran, diethyl carbonate,γ-butyrolactone, and solutes with quaternary ammonium salts or alkylammonium salts such as tetraethylammonium tetrafluoroborate (N(Et)₄BF₄)or triethyl (methyl) tetrafluoroborate (NMe(Et)₃BF₄). Organicelectrolytes are more expensive than aqueous electrolytes, but they havea higher dissociation voltage of typically 1.35 V per electrode (2.7 Vcapacitor voltage), and a higher temperature range. The lower electricalconductivity of organic solvents (10 to 60 mS/cm) leads to a lower powerdensity, but a higher energy density since the energy density isproportional to the square of the voltage.

The ionic liquid is composed of ions only. Ionic liquids are low meltingtemperature salts that are in a molten or liquid state when above adesired temperature. For instance, a salt is considered as an ionicliquid if its melting point is below 100° C. If the melting temperatureis equal to or lower than room temperature (25° C.), the salt isreferred to as a room temperature ionic liquid (RTIL). The IL salts arecharacterized by weak interactions, due to the combination of a largecation and a charge-delocalized anion. This results in a low tendency tocrystallize due to flexibility (anion) and asymmetry (cation).

A typical and well-known ionic liquid is formed by the combination of a1-ethyl-3-methylimidazolium (EMI) cation and anN,N-bis(trifluoromethane)sulphonamide (TFSI) anion. This combinationgives a fluid with an ionic conductivity comparable to many organicelectrolyte solutions and a low decomposition propensity and low vaporpressure up to ˜300-400° C. This implies a generally low volatility andnon-flammability and, hence, a much safer electrolyte for batteries.

Ionic liquids are basically composed of organic ions that come in anessentially unlimited number of structural variations owing to thepreparation ease of a large variety of their components. Thus, variouskinds of salts can be used to design the ionic liquid that has thedesired properties for a given application. These include, among others,imidazolium, pyrrolidinium and quaternary ammonium salts as cations andbis(trifluoromethanesulphonyl) imide, bis(fluorosulphonyl)imide, andhexafluorophosphate as anions. Based on their compositions, ionicliquids come in different classes that basically include aprotic, proticand zwitterionic types, each one suitable for a specific application.

Common cations of room temperature ionic liquids (RTILs) include, butnot limited to, tetraalkylammonium, di-, tri-, andtetra-alkylimidazolium, alkylpyridinium, dialkyl-pyrrolidinium,dialkylpiperidinium, tetraalkylphosphonium, and trialkylsulfonium.Common anions of RTILs include, but not limited to, BF₄ ⁻, B(CN)₄ ⁻,CH₃BF₃ ⁻, CH2CHBF₃ ⁻, CF₃BF₃ ⁻, C₂F₅BF₃ ⁻, n-C₃F₇BF₃ ⁻, n-C₄F₉BF₃ ⁻, PF₆⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, N(SO₂CF₃)₂ ⁻, N(COCF₃)(SO₂CF₃)⁻, N(SO₂F)₂ ⁻,N(CN)₂ ⁻, C(CN)₃ ⁻, SCN⁻, SeCN⁻, CuCl₂ ⁻, AlCl₄ ⁻, F(HF)_(2.3) ⁻, etc.Relatively speaking, the combination of imidazolium- or sulfonium-basedcations and complex halide anions such as AlCl₄ ⁻, BF₄ ⁻, CF₃CO₂ ⁻,CF₃SO₃ ⁻, NTf₂ ⁻, N(SO₂F)₂ ⁻, or F(HF)_(2.3) ⁻ results in RTILs withgood working conductivities.

RTILs can possess archetypical properties such as high intrinsic ionicconductivity, high thermal stability, low volatility, low (practicallyzero) vapor pressure, non-flammability, the ability to remain liquid ata wide range of temperatures above and below room temperature, highpolarity, high viscosity, and wide electrochemical windows. Theseproperties, except for the high viscosity, are desirable attributes whenit comes to using an RTIL as an electrolyte ingredient (a salt and/or asolvent) in a supercapacitor.

In order to make a pseudo-capacitor (a supercapacitor that works on thedevelopment of pseudo-capacitance through redox pair formation), theanode active material or cathode active material may be designed tocontain graphene sheets and a redox pair partner material selected froma metal oxide, a conducting polymer (e.g. conjugate-chain polymers), anon-conducting polymer (e.g. polyacrylonitrile, PAN), an organicmaterial (e.g. hydroquinone), a non-graphene carbon material, aninorganic material, or a combination thereof. Many of the materials thatcan pair up with reduced graphene oxide sheets are well-known in theart. In this study, we have come to realize that graphene halogenide(e.g. graphene fluoride), graphene hydrogenide, and nitrogenatedgraphene can work with a wide variety of partner materials to form aredox pair for developing pseudo-capacitance.

For instance, the metal oxide or inorganic materials that serve in sucha role include RuO₂, IrO₂, NiO, MnO₂, VO₂, V₂O₅, V₃O₈, TiO₂, Cr₂O₃,Co₂O₃, Co₃O₄, PbO₂, Ag₂O, MoC_(x), Mo₂N, or a combination thereof. Ingeneral, the inorganic material may be selected from a metal carbide,metal nitride, metal boride, metal dichalcogenide, or a combinationthereof. Preferably, the desired metal oxide or inorganic material isselected from an oxide, dichalcogenide, trichalcogenide, sulfide,selenide, or telluride of niobium, zirconium, molybdenum, hafnium,tantalum, tungsten, titanium, vanadium, chromium, cobalt, manganese,iron, or nickel in a nanowire, nano-disc, nano-ribbon, or nano plateletform. These materials or their precursors can be incorporated in thecoating slurry prior to the coating or film forming procedure.Alternatively, their molecular precursors in a liquid solution may beimpregnated into the pores of the graphene foam and the precursor isthen thermally or chemically converted into the desired inorganicspecies (e.g. transition metal oxide). The liquid or gel electrolyte isthen impregnated into the foam.

The following examples are used to illustrate some specific detailsabout the best modes of practicing the instant invention and should notbe construed as limiting the scope of the invention.

EXAMPLE 1 Various Blowing Agents and Pore-Forming (Bubble-Producing)Processes

In the field of plastic processing, chemical blowing agents are mixedinto the plastic pellets in the form of powder or pellets and dissolvedat higher temperatures. Above a certain temperature specific for blowingagent dissolution, a gaseous reaction product (usually nitrogen or CO₂)is generated, which acts as a blowing agent. However, a chemical blowingagent cannot be dissolved in a graphene material, which is a solid, notliquid. This presents a challenge to make use of a chemical blowingagent to generate pores or cells in a graphene material.

After extensive experimenting, we have discovered that practically anychemical blowing agent (e.g. in a powder or pellet form) can be used tocreate pores or bubbles in a dried layer of graphene when the first heattreatment temperature is sufficient to activate the blowing reaction.The chemical blowing agent (powder or pellets) may be dispersed in theliquid medium to become a second dispersed phase (sheets of graphenematerial being the first dispersed phase) in the suspension, which canbe deposited onto the solid supporting substrate to form a wet layer.This wet layer of graphene material may then be dried and heat treatedto activate the chemical blowing agent. After a chemical blowing agentis activated and bubbles are generated, the resulting foamed graphenestructure is largely maintained even when subsequently a higher heattreatment temperature is applied to the structure. This is quiteunexpected, indeed.

Chemical foaming agents (CFAs) can be organic or inorganic compoundsthat release gasses upon thermal decomposition. CFAs are typically usedto obtain medium- to high-density foams, and are often used inconjunction with physical blowing agents to obtain low-density foams.CFAs can be categorized as either endothermic or exothermic, whichrefers to the type of decomposition they undergo. Endothermic typesabsorb energy and typically release carbon dioxide and moisture upondecomposition, while the exothermic types release energy and usuallygenerate nitrogen when decomposed. The overall gas yield and pressure ofgas released by exothermic foaming agents is often higher than that ofendothermic types. Endothermic CFAs are generally known to decompose inthe range of 130 to 230° C. (266-446° F.), while some of the more commonexothermic foaming agents decompose around 200° C. (392° F.). However,the decomposition range of most exothermic CFAs can be reduced byaddition of certain compounds. The activation (decomposition)temperatures of CFAs fall into the range of our heat treatmenttemperatures. Examples of suitable chemical blowing agents includesodium bicarbonate (baking soda), hydrazine, hydrazide, azodicarbonamide(exothermic chemical blowing agents), nitroso compounds (e.g. N,N-Dinitroso pentamethylene tetramine), hydrazine derivatives (e.g. 4.4′-Oxybis (benzenesulfonyl hydrazide) and Hydrazo dicarbonamide), andhydrogen carbonate (e.g. Sodium hydrogen carbonate). These are allcommercially available in plastics industry.

In the production of foamed plastics, physical blowing agents aremetered into the plastic melt during foam extrusion or injection moldedfoaming, or supplied to one of the precursor materials duringpolyurethane foaming. It has not been previously known that a physicalblowing agent can be used to create pores in a graphene material, whichis in a solid state (not melt). We have surprisingly observed that aphysical blowing agent (e.g. CO₂ or N₂) can be injected into the streamof graphene suspension prior to being coated or cast onto the supportingsubstrate. This would result in a foamed structure even when the liquidmedium (e.g. water and/or alcohol) is removed. The dried layer ofgraphene material is capable of maintaining a controlled amount of poresor bubbles during liquid removal and subsequent heat treatments.

Technically feasible blowing agents include Carbon dioxide (CO₂),Nitrogen (N₂), Isobutane (C₄H₁₀), Cyclopentane (C₅H₁₀), Isopentane(C₅H₁₂), CFC-11 (CFCI₃), HCFC-22 (CHF₂CI), HCFC-142b (CF₂CICH₃), andHCFC-134a (CH₂FCF₃). However, in selecting a blowing agent,environmental safety is a major factor to consider. The MontrealProtocol and its influence on consequential agreements pose a greatchallenge for the producers of foam. Despite the effective propertiesand easy handling of the formerly applied chlorofluorocarbons, there wasa worldwide agreement to ban these because of their ozone depletionpotential (ODP). Partially halogenated chlorofluorocarbons are also notenvironmentally safe and therefore already forbidden in many countries.The alternatives are hydrocarbons, such as isobutane and pentane, andthe gases such as CO₂ and nitrogen.

Except for those regulated substances, all the blowing agents recitedabove have been tested in our experiments. For both physical blowingagents and chemical blowing agents, the blowing agent amount introducedinto the suspension is defined as a blowing agent-to-graphene materialweight ratio, which is typically from 0/1.0 to 1.0/1.0.

EXAMPLE 2 Preparation of Discrete Nano Graphene Platelets (NGPs) whichare GO Sheets

Chopped graphite fibers with an average diameter of 12 μm and naturalgraphite particles were separately used as a starting material, whichwas immersed in a mixture of concentrated sulfuric acid, nitric acid,and potassium permanganate (as the chemical intercalate and oxidizer) toprepare graphite intercalation compounds (GICs). The starting materialwas first dried in a vacuum oven for 24 h at 80° C. Then, a mixture ofconcentrated sulfuric acid, fuming nitric acid, and potassiumpermanganate (at a weight ratio of 4:1:0.05) was slowly added, underappropriate cooling and stirring, to a three-neck flask containing fibersegments. After 5-16 hours of reaction, the acid-treated graphite fibersor natural graphite particles were filtered and washed thoroughly withdeionized water until the pH level of the solution reached 6. Afterbeing dried at 100° C. overnight, the resulting graphite intercalationcompound (GIC) or graphite oxide fiber was re-dispersed in water and/oralcohol to form a slurry.

In one sample, five grams of the graphite oxide fibers were mixed with2,000 ml alcohol solution consisting of alcohol and distilled water witha ratio of 15:85 to obtain a slurry mass. Then, the mixture slurry wassubjected to ultrasonic irradiation with a power of 200 W for variouslengths of time. After 20 minutes of sonication, GO fibers wereeffectively exfoliated and separated into thin graphene oxide sheetswith oxygen content of approximately 23%-31% by weight. The resultingsuspension contains GO sheets being suspended in water. A chemicalblowing agent (hydrazo dicarbonamide) was added to the suspension justprior to casting.

The resulting suspension was then cast onto a glass surface using adoctor's blade to exert shear stresses, inducing GO sheet orientations.The resulting GO coating films, after removal of liquid, have athickness that can be varied from approximately 5 to 500 μm (preferablyand typically from 10 μm to 50 μm).

For making a graphene foam specimen, the GO coating film was thensubjected to heat treatments that typically involve an initial thermalreduction temperature of 80-350° C. for 1-8 hours, followed byheat-treating at a second temperature of 1,500-2,850° C. for 0.5 to 5hours. It may be noted that we have found it essential to apply acompressive stress to the coating film sample while being subjected tothe first heat treatment. This compress stress seems to have helpedmaintain good contacts between the graphene sheets so that chemicalmerging and linking between graphene sheets can occur while pores arebeing formed. Without such a compressive stress, the heat-treated filmis typically excessively porous with constituent graphene sheets in thepore walls being very poorly oriented and incapable of chemical mergingand linking with one another. As a result, the thermal conductivity,electrical conductivity, and mechanical strength of the graphene foamare severely compromised.

EXAMPLE 3 Preparation of Single-Layer Graphene Sheets from Meso-CarbonMicro-Beads (MCMBs)

Meso-carbon microbeads (MCMBs) were supplied from China Steel ChemicalCo., Kaohsiung, Taiwan. This material has a density of about 2.24 g/cm³with a median particle size of about 16 μm. MCMB (10 grams) wereintercalated with an acid solution (sulfuric acid, nitric acid, andpotassium permanganate at a ratio of 4:1:0.05) for 48-96 hours. Uponcompletion of the reaction, the mixture was poured into deionized waterand filtered. The intercalated MCMBs were repeatedly washed in a 5%solution of HCl to remove most of the sulphate ions. The sample was thenwashed repeatedly with deionized water until the pH of the filtrate wasno less than 4.5. The slurry was then subjected ultrasonication for10-100 minutes to produce GO suspensions. TEM and atomic forcemicroscopic studies indicate that most of the GO sheets weresingle-layer graphene when the oxidation treatment exceeded 72 hours,and 2- or 3-layer graphene when the oxidation time was from 48 to 72hours.

The GO sheets contain oxygen proportion of approximately 35%-47% byweight for oxidation treatment times of 48-96 hours. GO sheets weresuspended in water. Baking soda (5-20% by weight), as a chemical blowingagent, was added to the suspension just prior to casting. The suspensionwas then cast onto a glass surface using a doctor's blade to exert shearstresses, inducing GO sheet orientations. Several samples were cast,some containing a blowing agent and some not. The resulting GO films,after removal of liquid, have a thickness that can be varied fromapproximately 10 to 500 μm.

The several sheets of the GO film, with or without a blowing agent, werethen subjected to heat treatments that involve an initial (first)thermal reduction temperature of 80-500° C. for 1-5 hours. This firstheat treatment generated a graphene foam. However, the graphene domainsin the foam wall can be further perfected (re-graphitized to become moreordered or having a higher degree of crystallinity and larger lateraldimensions of graphene planes, longer than the original graphene sheetdimensions due to chemical merging) if the foam is followed byheat-treating at a second temperature of 1,500-2,850° C.

EXAMPLE 4 Preparation of Pristine Graphene Foam (0% Oxygen)

Recognizing the possibility of the high defect population in GO sheetsacting to reduce the conductivity of individual graphene plane, wedecided to study if the use of pristine graphene sheets (non-oxidizedand oxygen-free, non-halogenated and halogen-free, etc.) can lead to agraphene foam having a higher thermal conductivity. Pristine graphenesheets were produced by using the direct ultrasonication or liquid-phaseproduction process.

In a typical procedure, five grams of graphite flakes, ground toapproximately 20 μm or less in sizes, were dispersed in 1,000 mL ofdeionized water (containing 0.1% by weight of a dispersing agent, Zonyl®FSO from DuPont) to obtain a suspension. An ultrasonic energy level of85 W (Branson S450 Ultrasonicator) was used for exfoliation, separation,and size reduction of graphene sheets for a period of 15 minutes to 2hours. The resulting graphene sheets are pristine graphene that havenever been oxidized and are oxygen-free and relatively defect-free.There are no other non-carbon elements.

Various amounts (1%-30% by weight relative to graphene material) ofchemical bowing agents (N, N-Dinitroso pentamethylene tetramine or 4.4′-Oxybis (benzenesulfonyl hydrazide) were added to a suspensioncontaining pristine graphene sheets and a surfactant. The suspension wasthen cast onto a glass surface using a doctor's blade to exert shearstresses, inducing graphene sheet orientations. Several samples werecast, including one that was made using CO2 as a physical blowing agentintroduced into the suspension just prior to casting). The resultinggraphene films, after removal of liquid, have a thickness that can bevaried from approximately 10 to 100 μm.

The graphene films were then subjected to heat treatments that involvean initial (first) thermal reduction temperature of 80-1,500° C. for 1-5hours. This first heat treatment generated a graphene foam. Some of thepristine foam samples were then subjected to a second temperature of1,500-2,850° C. to determine if the graphene domains in the foam wallcould be further perfected (re-graphitized to become more ordered orhaving a higher degree of crystallinity).

COMPARATIVE EXAMPLE 4-a CVD Graphene Foams on Ni Foam Templates

The procedure was adapted from that disclosed in open literature: Chen,Z. et al. “Three-dimensional flexible and conductive interconnectedgraphene networks grown by chemical vapor deposition,” Nat. Mater. 10,424-428 (2011). Nickel foam, a porous structure with an interconnected3D scaffold of nickel was chosen as a template for the growth ofgraphene foam. Briefly, carbon was introduced into a nickel foam bydecomposing CH₄ at 1,000° C. under ambient pressure, and graphene filmswere then deposited on the surface of the nickel foam. Due to thedifference in the thermal expansion coefficients between nickel andgraphene, ripples and wrinkles were formed on the graphene films. Inorder to recover (separate) graphene foam, Ni frame must be etched away.Before etching away the nickel skeleton by a hot HCl (or FeCl₃)solution, a thin layer of poly(methyl methacrylate) (PMMA) was depositedon the surface of the graphene films as a support to prevent thegraphene network from collapsing during nickel etching. After the PMMAlayer was carefully removed by hot acetone, a fragile graphene foamsample was obtained. The use of the PMMA support layer is critical topreparing a free-standing film of graphene foam; only a severelydistorted and deformed graphene foam sample was obtained without thePMMA support layer. This is a tedious process that is notenvironmentally benign and is not scalable.

COMPARATIVE EXAMPLE 4-b Conventional Graphitic Foam from Pitch-BasedCarbon Foams

Pitch powder, granules, or pellets are placed in a aluminum mold withthe desired final shape of the foam. Mitsubishi ARA-24 meso-phase pitchwas utilized. The sample is evacuated to less than 1 torr and thenheated to a temperature approximately 300° C. At this point, the vacuumwas released to a nitrogen blanket and then a pressure of up to 1,000psi was applied. The temperature of the system was then raised to 800°C. This was performed at a rate of 2 degree C./min. The temperature washeld for at least 15 minutes to achieve a soak and then the furnacepower was turned off and cooled to room temperature at a rate ofapproximately 1.5 degree C./min with release of pressure at a rate ofapproximately 2 psi/min. Final foam temperatures were 630° C. and 800°C. During the cooling cycle, pressure is released gradually toatmospheric conditions. The foam was then heat treated to 1050° C.(carbonized) under a nitrogen blanket and then heat treated in separateruns in a graphite crucible to 2500° C. and 2800° C. (graphitized) inArgon.

Samples from the foam were machined into specimens for measuring thethermal conductivity. The bulk thermal conductivity ranged from 67 W/mKto 151 W/mK. The density of the samples was from 0.31-0.61 g/cm³. Whenweight is taken into account, the specific thermal conductivity of thepitch derived foam is approximately 67/0.31=216 and 151/0.61=247.5 W/mKper specific gravity (or per physical density).

The compression strength of the samples having an average density of0.51 g/cm³ was measured to be 3.6 MPa and the compression modulus wasmeasured to be 74 MPa. By contrast, the compression strength andcompressive modulus of the presently invented graphene foam samplesderived from GO having a comparable physical density are 5.7 MPa and 103MPa, respectively.

Shown in FIG. 4(A) and FIG. 5(A) are the thermal conductivity values vs.specific gravity of the GO suspension-derived foam, meso-phasepitch-derived graphite foam, and Ni foam template-assisted CVD graphenefoam. These data clearly demonstrate the following unexpected results:

-   -   1) GO-derived graphene foams produced by the presently invented        process exhibit significantly higher thermal conductivity as        compared to both meso-phase pitch-derived graphite foam and Ni        foam template-assisted CVD graphene, given the same physical        density.    -   2) This is quite surprising in view of the notion that CVD        graphene is essentially pristine graphene that has never been        exposed to oxidation and should have exhibited a much higher        thermal conductivity compared to graphene oxide (GO). GO is        known to be highly defective (having a high defect population        and, hence, low conductivity) even after the oxygen-containing        functional groups are removed via conventional thermal or        chemical reduction methods. These exceptionally high thermal        conductivity values observed with the GO-derived graphene foams        herein produced are much to our surprise.    -   3) FIG. 5(A) presents the thermal conductivity values over        comparable ranges of specific gravity values to allow for        calculation of specific conductivity (conductivity value, W/mK,        divided by physical density value, g/cm³) for all three        graphitic foam materials based on the slopes of the curves        (approximately straight lines at different segments). These        specific conductivity values enable a fair comparison of thermal        conductivity values of these three types of graphitic foams        given the same amount of solid graphitic material in each foam.        These data provide an index of the intrinsic conductivity of the        solid portion of the foam material. These data clearly indicate        that, given the same amount of solid material, the presently        invented GO-derived foam is intrinsically most conducting,        reflecting a high level of graphitic crystal perfection (larger        crystal dimensions, fewer grain boundaries and other defects,        better crystal orientation, etc.). This is also unexpected.    -   4) The specific conductivity values of the presently invented        GO- and GF-derived foam exhibit values from 250 to 500 W/mK per        unit of specific gravity; but those of the other two foam        materials are typically lower than 250 W/mK per unit of specific        gravity.

Summarized in FIG. 7 are thermal conductivity data for a series ofGO-derived graphene foams and a series of pristine graphene derivedfoams, both plotted over the final (maximum) heat treatmenttemperatures. These data indicate that the thermal conductivity of theGO foams is highly sensitive to the final heat treatment temperature(HTT). Even when the HTT is very low, clearly some type of graphenemerging or crystal perfection reactions are already activated. Thethermal conductivity increases monotonically with the final HTT. Incontrast, the thermal conductivity of pristine graphene foams remainsrelatively constant until a final HTT of approximately 2,500° C. isreached, signaling the beginning of a re-crystallization and perfectionof graphite crystals. There are no functional groups in pristinegraphene, such as —COOH in GO, that enable chemical linking of graphenesheets at relatively low HTTs. With a HTT as low as 1,250° C., GO sheetscan merge to form significantly larger graphene sheets with reducedgrain boundaries and other defects. Even though GO sheets areintrinsically more defective than pristine graphene, the presentlyinvented process enables the GO sheets to form graphene foams thatoutperform pristine graphene foams. This is another unexpected result.

EXAMPLE 5 Preparation of Graphene Oxide (GO) Suspension from NaturalGraphite and of Subsequent GO Foams

Graphite oxide was prepared by oxidation of graphite flakes with anoxidizer liquid consisting of sulfuric acid, sodium nitrate, andpotassium permanganate at a ratio of 4:1:0.05 at 30° C. When naturalgraphite flakes (particle sizes of 14 μm) were immersed and dispersed inthe oxidizer mixture liquid for 48 hours, the suspension or slurryappears and remains optically opaque and dark. After 48 hours, thereacting mass was rinsed with water 3 times to adjust the pH value to atleast 3.0. A final amount of water was then added to prepare a series ofGO-water suspensions. We observed that GO sheets form a liquid crystalphase when GO sheets occupy a weight fraction >3% and typically from 5%to 15%.

By dispensing and coating the GO suspension on a polyethyleneterephthalate (PET) film in a slurry coater and removing the liquidmedium from the coated film we obtained a thin film of dried grapheneoxide. Several GO film samples were then subjected to different heattreatments, which typically include a thermal reduction treatment at afirst temperature of 100° C. to 500° C. for 1-10 hours, and at a secondtemperature of 1,500° C.-2,850° C. for 0.5-5 hours. With these heattreatments, also under a compressive stress, the GO films weretransformed into graphene foam.

COMPARATIVE EXAMPLE 5-a Graphene Foams from Hydrothermally ReducedGraphene Oxide

For comparison, a self-assembled graphene hydrogel (SGH) sample wasprepared by a one-step hydrothermal method. In a typical procedure, theSGH can be easily prepared by heating 2 mg/mL of homogeneous grapheneoxide (GO) aqueous dispersion sealed in a Teflon-lined autoclave at 180°C. for 12 h. The SGH containing about 2.6% (by weight) graphene sheetsand 97.4% water has an electrical conductivity of approximately 5×10⁻³S/cm. Upon drying and heat treating at 1,500° C., the resulting graphenefoam exhibits an electrical conductivity of approximately 1.5×10⁻¹ S/cm,which is 2 times lower than those of the presently invented graphenefoams produced by heat treating at the same temperature.

COMPARATIVE EXAMPLE 5-b Plastic Bead Template-Assisted Formation ofReduced Graphene Oxide Foams

A hard template-directed ordered assembly for a macro-porous bubbledgraphene film (MGF) was prepared. Mono-disperse poly methyl methacrylate(PMMA) latex spheres were used as the hard templates. The GO liquidcrystal prepared in Example 5 was mixed with a PMMA spheres suspension.Subsequent vacuum filtration was then conducted to prepare the assemblyof PMMA spheres and GO sheets, with GO sheets wrapped around the PMMAbeads. A composite film was peeled off from the filter, air dried andcalcinated at 800° C. to remove the PMMA template and thermally reduceGO into RGO simultaneously. The grey free-standing PMMA/GO film turnedblack after calcination, while the graphene film remained porous.

FIG. 4(B) and FIG. 5(B) show the thermal conductivity values of thepresently invented GO suspension-derived foam, GO foam produced viasacrificial plastic bead template-assisted process, and hydrothermallyreduced GO graphene foam. Most surprisingly, given the same starting GOsheets, the presently invented process produces the highest-performinggraphene foams. Electrical conductivity data summarized in FIG. 4(C) arealso consistent with this conclusion. These data further support thenotion that, given the same amount of solid material, the presentlyinvented GO suspension deposition (with stress-induced orientation) andsubsequent heat treatments give rise to a graphene foam that isintrinsically most conducting, reflecting a highest level of graphiticcrystal perfection (larger crystal dimensions, fewer grain boundariesand other defects, better crystal orientation, etc. along the porewalls).

It is of significance to point out that all the prior art processes forproducing graphite foams or graphene foams appear to providemacro-porous foams having a physical density in the range ofapproximately 0.2-0.6 g/cm³ only with pore sizes being typically toolarge (e.g. from 20 to 300 μm) for most of the intended applications. Incontrast, the instant invention provides processes that generategraphene foams having a density that can be as low as 0.01 g/cm³ and ashigh as 1.7 g/cm³. The pore sizes can be varied between meso-scaled(2-50 nm) up to macro-scaled (1-500 μm) depending upon the contents ofnon-carbon elements and the amount/type of blowing agent used. Thislevel of flexibility and versatility in designing various types ofgraphene foams is unprecedented and un-matched by any prior art process.

EXAMPLE 6 Preparation of Graphene Foams from Graphene Fluoride

Several processes have been used by us to produce GF, but only oneprocess is herein described as an example. In a typical procedure,highly exfoliated graphite (HEG) was prepared from intercalated compoundC₂F.xClF₃. HEG was further fluorinated by vapors of chlorine trifluorideto yield fluorinated highly exfoliated graphite (FHEG). Pre-cooledTeflon reactor was filled with 20-30 mL of liquid pre-cooled ClF₃, thereactor was closed and cooled to liquid nitrogen temperature. Then, nomore than 1 g of HEG was put in a container with holes for ClF₃ gas toaccess and situated inside the reactor. In 7-10 days a gray-beigeproduct with approximate formula C₂F was formed.

Subsequently, a small amount of FHEG (approximately 0.5 mg) was mixedwith 20-30 mL of an organic solvent (methanol, ethanol, 1-propanol,2-propanol, 1-butanol, tert-butanol, isoamyl alcohol) and subjected toan ultrasound treatment (280 W) for 30 min, leading to the formation ofhomogeneous yellowish dispersions. Five minutes of sonication was enoughto obtain a relatively homogenous dispersion, but longer sonicationtimes ensured better stability. Upon casting on a glass surface with thesolvent removed, the dispersion became a brownish film formed on theglass surface. When GF films were heat-treated, fluorine was released asgases that helped to generate pores in the film. In some samples, aphysical blowing agent (N₂ gas) was injected into the wet GF film whilebeing cast. These samples exhibit much higher pore volumes or lower foamdensities. Without using a blowing agent, the resulting graphenefluoride foams exhibit physical densities from 0.35 to 1.38 g/cm³. Whena blowing agent was used (blowing agent/GF weight ratio from 0.5/1 to0.05/1), a density from 0.02 to 0.35 g/cm³ was obtained. Typicalfluorine contents are from 0.001% (HTT=2,500° C.) to 4.7% (HTT=350° C.),depending upon the final heat treatment temperature involved.

FIG. 6 presents a comparison in thermal conductivity values of thegraphene foam samples derived from GO and GF (graphene fluoride),respectively, as a function of the specific gravity. It appears that theGF foams, in comparison with GO foams, exhibit higher thermalconductivity values at comparable specific gravity values. Both deliverimpressive heat-conducting capabilities, being the best among all knownfoamed materials.

EXAMPLE 7 Preparation of Graphene Foams from Nitrogenated Graphene

Graphene oxide (GO), synthesized in Example 2, was finely ground withdifferent proportions of urea and the pelletized mixture heated in amicrowave reactor (900 W) for 30 s. The product was washed several timeswith deionized water and vacuum dried. In this method graphene oxidegets simultaneously reduced and doped with nitrogen. The productsobtained with graphene:urea mass ratios of 1:0.5, 1:1 and 1:2 aredesignated as NGO-1, NGO-2 and NGO-3 respectively and the nitrogencontents of these samples were 14.7, 18.2 and 17.5 wt % respectively asfound by elemental analysis. These nitrogenated graphene sheets remaindispersible in water. The resulting suspensions were then cast, dried,and heat-treated initially at 200-350° C. as a first heat treatmenttemperature and subsequently treated at a second temperature of 1,500°C. The resulting nitrogenated graphene foams exhibit physical densitiesfrom 0.45 to 1.28 g/cm³. Typical nitrogen contents of the foams are from0.01% (HTT=1,500° C.) to 5.3% (HTT=350° C.), depending upon the finalheat treatment temperature involved.

EXAMPLE 8 Characterization of Various Graphene Foams and ConventionalGraphite Foam

The internal structures (crystal structure and orientation) of severaldried GO layers, and the heat-treated films at different stages of heattreatments were investigated using X-ray diffraction. The X-raydiffraction curve of natural graphite typically exhibits a peak atapproximately 2θ=26°, corresponds to an inter-graphene spacing (d₀₀₂) ofapproximately 0.3345 nm. Upon oxidation, the resulting GO shows an X-raydiffraction peak at approximately 2θ=12°, which corresponds to aninter-graphene spacing (d₀₀₂) of approximately 0.7 nm. With some heattreatment at 150° C., the dried GO compact exhibits the formation of ahump centered at 22°, indicating that it has begun the process ofdecreasing the inter-graphene spacing due to the beginning of chemicallinking and ordering processes. With a heat treatment temperature of2,500° C. for one hour, the d₀₀₂ spacing has decreased to approximately0.336, close to 0.3354 nm of a graphite single crystal.

With a heat treatment temperature of 2,750° C. for one hour, the d₀₀₂spacing is decreased to approximately to 0.3354 nm, identical to that ofa graphite single crystal. In addition, a second diffraction peak with ahigh intensity appears at 2θ=55° corresponding to X-ray diffraction from(004) plane. The (004) peak intensity relative to the (002) intensity onthe same diffraction curve, or the I(004)/I(002) ratio, is a goodindication of the degree of crystal perfection and preferred orientationof graphene planes. The (004) peak is either non-existing or relativelyweak, with the I(004)/I(002) ratio <0.1, for all graphitic materialsheat treated at a temperature lower than 2,800° C. The I(004)/I(002)ratio for the graphitic materials heat treated at 3,000-3,250° C. (e,g,highly oriented pyrolytic graphite, HOPG) is in the range of 0.2-0.5. Incontrast, a graphene foam prepared with a final HTT of 2,750° C. for onehour exhibits a I(004)/I(002) ratio of 0.78 and a Mosaic spread value of0.21, indicating a practically perfect graphene single crystal with agood degree of preferred orientation.

The “mosaic spread” value is obtained from the full width at halfmaximum of the (002) reflection in an X-ray diffraction intensity curve.This index for the degree of ordering characterizes the graphite orgraphene crystal size (or grain size), amounts of grain boundaries andother defects, and the degree of preferred grain orientation. A nearlyperfect single crystal of graphite is characterized by having a mosaicspread value of 0.2-0.4. Some of our graphene foams have a mosaic spreadvalue in this range of 0.2-0.4 when produced using a final heattreatment temperature no less than 2,500° C.

The inter-graphene spacing values of both the GO suspension-derivedsamples obtained by heat treating at various temperatures over a widetemperature range are summarized in FIG. 8(A). Corresponding oxygencontent values in the GO suspension-derived unitary graphene layer areshown in FIG. 8(B).

It is of significance to point out that a heat treatment temperature aslow as 500° C. is sufficient to bring the average inter-graphene spacingin GO sheets along the pore walls to below 0.4 nm, getting closer andcloser to that of natural graphite or that of a graphite single crystal.The beauty of this approach is the notion that this GO suspensionstrategy has enabled us to re-organize, re-orient, and chemically mergethe planar graphene oxide molecules from originally different graphiteparticles or graphene sheets into a unified structure with all thegraphene planes now being larger in lateral dimensions (significantlylarger than the length and width of the graphene planes in the originalgraphite particles). A potential chemical linking mechanism isillustrated in FIG. 3. This has given rise to exceptional thermalconductivity and electrical conductivity values.

EXAMPLE 9 Preparation of Intrinsically Conductive Polymer-Graphene RedoxPairs in a Graphene

In this series of examples, intrinsically conductive polymers (e.g.polyaniline, poly polypyrrole, and polythiophene) and their sulfonatedversions are evaluated for their effectiveness as a redox pair partnermaterial with a graphene material.

The chemical synthesis of the sulfonated polyaniline (S-PANi) wasaccomplished by reacting polyaniline with concentrated sulfuric acid.The procedure was similar to that used by Epstein, et al. (U.S. Pat. No.5,109,070, Apr. 28, 1992). The resulting S-PANi can be represented bythe following Formula 1, with R₁, R₂, R₃, and R₄ group being H, SO₃ ⁻ orSO₃H (R₅=H) with the content of the latter two being varied between 30%and 75% (i.e., the degree of sulfonation varied between 30% and 75%).

The electron conductivity of these SO₃ ⁻ or SO₃H-based S-PANicompositions in the range of 0.1 S/cm to 0.5 S/cm when the degree ofsulfonation was from approximately 30% to 75% (with y beingapproximately 0.4-0.6). The S-PANi/water solution was impregnated into asolid graphene foam and, upon water removal, the S-PANi is precipitatedout and coated onto the graphene-based pore walls for forming a redox orpseudo-capacitance electrode. Non-sulfonated polymers can be dissolvedin select organic solvents.

A sulfonated pyrrole-based polymer (with X=NH and Y=SO₃ ⁻, m=1, and A=Hin the following formula) was synthesized by following a procedureadapted from Aldissi, et al., U.S. Pat. No. 4,880,508, Nov. 14, 1989.

For solution impregnation, as one example, approximately 5.78 g of theresulting sulfonated polypyrrole was dissolved in 100 ml of distilledwater. Then, the aqueous solution was impregnated into pores of agraphene foam and dried to allow for precipitation and deposition ofsulfonated polypyrrole onto surfaces of graphene-based pore walls toform a redox pair with graphene in the pore walls.

Water-soluble conductive polymers having a thiophene ring (X=sulfur) andalkyl groups containing 4 carbon atoms (m=4) in the above Formula 2 wereprepared, according to a method adapted from Aldissi, et al. (U.S. Pat.No. 4,880,508, Nov. 14, 1989). The surfactant molecules of thesepolymers were sulfonate groups with sodium. Conductivity of this polymerin a self-doped state was found to be from about 10⁻³ to about 10⁻²S/cm. When negative ions from a supporting electrolyte used duringsynthesis were allowed to remain in the polymer, conductivities up toabout 50 S/cm were observed.

A doped poly (alkyl thiophene) (PAT) with Y=SO₃H and A=H in Formula 2that exhibited an electron conductivity of 12.5 S/cm was dissolved in anaqueous hydrogen peroxide (H₂O₂) solution. The resulting polymersolution was impregnated into a solid graphene foam and dried to form aredox or pseudo-capacitance electrode.

We have surprisingly discovered that the sulfonated version (e.g.S-PANi) of an intrinsically conductive polymer (PANi), when paired upwith a graphene foam material, leads to a significantly higherpseudo-capacitance value when compared with the un-sulfonated one; e.g.654 F/g (S-PANi) vs. 463 F/g (PANi) and 487 F/cm³ (S-PPy) vs. 355 F/cm³(PPy).

EXAMPLE 10 Preparation of MnO₂-Graphene Redox Pairs in a Graphene Foam

The MnO₂ powder was synthesized by two methods (one with the presence ofpristine graphene and the other within the pores of graphene-carbonfoam). In one method, a 0.1 mol/L KMnO₄ aqueous solution was prepared bydissolving potassium permanganate in deionized water. Meanwhile 13.3 gsurfactant of high purity sodium bis(2-ethylhexyl) sulfosuccinate wasadded in 300 mL iso-octane (oil) and stirred well to obtain an opticallytransparent solution. Then, 32.4 mL of 0.1 mol/L KMnO₄ solution wereadded in the solution, and a piece of graphene-carbon hybrid foam wasimmersed in the solution. On a separate basis, pristine graphene sheetswere added into the solution. The two pots of resulting suspensions wereultrasonicated for 30 min and a dark brown precipitate of MnO₂ wascoated on surfaces of the foam walls and graphene sheets, respectively.The products were recovered, washed several times with distilled waterand ethanol, and dried at 80° C. for 12 h. The samples were (1) agraphene-carbon hybrid foam structure having graphene wall-supportedMnO₂ and (2) MnO₂-coated graphene sheets, which were then packed into aporous paper-like structure using the vacuum-assisted filtration method.In both cases, graphene and MnO₂ form a redox pair operating to producepseudo-capacitance when a liquid electrolyte is impregnated into poresof the foam. With comparable electrode thickness (approximately 105 μm),the graphene foam-based electrode exhibits a significantly lowerequivalent series resistance.

EXAMPLE 11 Evaluation of Various Supercapacitor Cells

In most of the examples investigated, both the inventive supercapacitorcells and their conventional counterparts were fabricated and evaluated.The latter cells, for comparison purposes, were prepared by theconventional procedures of slurry coating of electrodes, drying ofelectrodes, assembling of anode layer, separator, and cathode layer,packaging of assembled laminate, and injection of liquid electrolyte. Ina conventional cell, an electrode (cathode or anode), is typicallycomposed of 85% an electrode active material (e.g. graphene, activatedcarbon, inorganic nano discs, etc.), 5% Super-P (acetylene black-basedconductive additive), and 10% PTFE, which were mixed and coated on Alfoil. The thickness of electrode is around 100 μm. For each sample, bothcoin-size and pouch cells were assembled in a glove box. The capacitywas measured with galvanostatic experiments using an Arbin SCTSelectrochemical testing instrument. Cyclic voltammetry (CV) andelectrochemical impedance spectroscopy (EIS) were conducted on anelectrochemical workstation (CHI 660 System, USA).

Galvanostatic charge/discharge tests were conducted on the samples toevaluate the electrochemical performance. For the galvanostatic tests,the specific capacity (q) is calculated asq=I*t/m  (1)where I is the constant current in mA, t is the time in hours, and m isthe cathode active material mass in grams. With voltage V, the specificenergy (E) is calculated as,E=∫Vdq  (2)The specific power (P) can be calculated asP=(E/t)(W/kg)  (3)where t is the total charge or discharge step time in hours.The specific capacitance (C) of the cell is represented by the slope ateach point of the voltage vs. specific capacity plot,C=dq/dV  (4)For each sample, several current density (representing charge/dischargerates) were imposed to determine the electrochemical responses, allowingfor calculations of energy density and power density values required ofthe construction of a Ragone plot (power density vs. energy density).

EXAMPLE 12 Achievable Electrode Tap Density and its Effect onElectrochemical Performance of Supercapacitor Cells

The presently invented process allows us to prepare graphene foam of anypractical tap density from 0.1 to 1.3 g/cm³. Tap densities higher than1.3 g/cm³ are possible, but the spaces between graphene sheets becometoo limited to accommodate a sufficient amount of electrolyte. It may benoted that the graphene-based supercapacitor electrodes prepared byconventional processes are limited to <0.3 and mostly <0.2 g/cm³.Furthermore, as discussed earlier, only thinner electrodes can beprepared using these conventional processes. As a point of reference,the activated carbon-based electrode exhibits a tap density typicallyfrom 0.3 to 0.5 g/cm³.

A series of EDLC electrodes with varying tap densities were preparedfrom the same batch of graphene foam sheet, but roll-pressed todifferent extents. The volume and weights of an electrode were measuredbefore and after electrolyte impregnation and before and afterroll-pressing. These measurements enabled us to estimate the tap densityof the dried electrode (wet electrode volume/weight minus the amount ofelectrolyte actually absorbed). For comparison purposes, graphene-basedelectrodes of comparable thickness (70-75 μm) were also prepared usingthe conventional slurry coating process (the wet-dry-wet procedures).The electrode specific capacitance values of these supercapacitors usingan organic electrolyte (acetonitrile) are summarized in FIG. 9. Thereare several significant observations that can be made from these data:

-   -   (A) Given comparable electrode thickness, the presently invented        graphene foam electrodes exhibit significantly higher        gravimetric specific capacitance (185-289 F/g) as compared to        those (131-145 F/g) of the corresponding graphene-based        electrodes prepared by the conventional process, all based on        EDLC alone.    -   (B) The highest achievable tap density of the electrode prepared        by the conventional method is 0.14-0.28 g/cm³. In contrast, the        presently invented process makes it possible to achieve a tap        density of 0.35-1.61 g/cm3; these unprecedented values even        surpass those (0.3-0.5 g/cm³) of activated carbon electrodes by        a large margin.    -   (C) The presently invented graphene foam electrodes exhibit a        volumetric specific capacitance up to 338 F/cm³, which is also        an unprecedented value. In contrast, the graphene electrodes        prepared according to the conventional method shows a specific        capacitance in the range of 20-37 F/cm³; the differences are        dramatic.

Shown in FIG. 10 are Ragone plots (gravimetric and volumetric powerdensity vs. energy density) of two sets of symmetric supercapacitor(EDLC) cells containing nitrogen-doped graphene sheets andnitrogen-doped graphene foam as the electrode active material andEMIMBF4 ionic liquid as the electrolyte. One of the two series ofsupercapacitors was based on the graphene foam electrode preparedaccording to an embodiment of instant invention and the other was by theconventional slurry coating of electrodes. Several significantobservations can be made from these data:

-   -   (A) Both the gravimetric and volumetric energy densities and        power densities of the supercapacitor cells prepared by the        presently invented method (denoted as “inventive” in the figure        legend) are significantly higher than those of their        counterparts prepared via the conventional method (denoted as        “conventional”). The differences are highly dramatic and are        mainly due to the high active material mass loading (>20 mg/cm²)        associated with the presently invented cells, reduced proportion        of overhead components (non-active) relative to the active        material weight/volume, no binder resin, the ability of the        inventive method to more effectively pack graphene sheets        together without graphene sheet re-stacking (pre-impregnated        electrolyte serving as a spacer).    -   (B) For the cells prepared by the conventional method, the        absolute magnitudes of the volumetric energy densities and        volumetric power densities are significantly lower than those of        their gravimetric energy densities and gravimetric power        densities, due to the very low tap density (packing density of        0.28 g/cm³) of isolated graphene sheet-based electrodes prepared        by the conventional slurry coating method.    -   (C) In contrast, for the cells prepared by the presently        invented method, the absolute magnitudes of the volumetric        energy densities and volumetric power densities are higher than        those of their gravimetric energy densities and gravimetric        power densities, due to the relatively high tap density (packing        density of 1.2 g/cm³) of graphene foam-based electrodes prepared        by the presently invented method.

Shown in FIG. 11 are Ragone plots of lithium ion capacitor (LIC) cellscontaining pristine graphene sheets as the cathode active material,prelithiated graphite particles as the anode active material, andlithium salt (LiPF₆)—PC/DEC as organic liquid electrolyte. The data arefor both LICs, wherein the cathode is prepared by the presently inventedelectrolyte-impregnated graphene foam method and those by theconventional slurry coating of electrodes. These data indicate that boththe gravimetric and volumetric energy densities and power densities ofthe LIC cells prepared by the presently invented method aresignificantly higher than those of their counterparts prepared via theconventional method. Again, the differences are huge and are mainlyascribed to the high active material mass loading (>25 mg/cm² at thecathode side) associated with the presently invented cells, reducedproportion of overhead (non-active) components relative to the activematerial weight/volume, no binder resin, the ability of the inventivemethod to more effectively pack graphene sheets together withoutre-stacking.

For the LIC cells prepared by the conventional method, the absolutemagnitudes of the volumetric energy densities and volumetric powerdensities are significantly lower than those of their gravimetric energydensities and gravimetric power densities, due to the very low tapdensity (packing density of 0.28 g/cm³) of pristine graphene-basedcathodes prepared by the conventional slurry coating method. Incontrast, for the LIC cells prepared by the instant method, the absolutemagnitudes of the volumetric energy densities and volumetric powerdensities are higher than those of their gravimetric energy densitiesand gravimetric power densities, due to the relatively high tap densityof pristine graphene-based cathodes prepared by the presently inventedmethod.

It is of significance to point out that reporting the energy and powerdensities per weight of active material alone on a Ragone plot, as didby many researchers, may not give a realistic picture of the performanceof the assembled supercapacitor cell. The weights of other devicecomponents also must be taken into account. These overhead components,including current collectors, electrolyte, separator, binder,connectors, and packaging, are non-active materials and do notcontribute to the charge storage amounts. They only add weights andvolumes to the device. Hence, it is desirable to reduce the relativeproportion of overhead component weights and increase the activematerial proportion. However, it has not been possible to achieve thisobjective using conventional supercapacitor production processes. Thepresent invention overcomes this long-standing, most serious problem inthe art of supercapacitors.

In a commercial supercapacitor having an electrode thickness of 150 μm,the weight of the active material (i.e. activated carbon) accounts forabout 30% of the total mass of the packaged cell. Hence, a factor of 3to 4 is frequently used to extrapolate the energy or power densities ofthe device (cell) from the properties based on the active materialweight alone. In most of the scientific papers, the properties reportedare typically based on the active material weight alone and theelectrodes are typically very thin (<<100 μm, and mostly <<50 μm). Theactive material weight is typically from 5% to 10% of the total deviceweight, which implies that the actual cell (device) energy or powerdensities may be obtained by dividing the corresponding active materialweight-based values by a factor of 10 to 20. After this factor is takeninto account, the properties reported in these papers do not really lookany better than those of commercial supercapacitors. Thus, one must bevery careful when it comes to read and interpret the performance data ofsupercapacitors reported in the scientific papers and patentapplications.

EXAMPLE 13 Achievable Electrode Thickness and its Effect onElectrochemical Performance of Supercapacitor Cells

One might be tempted to think the electrode thickness of asupercapacitor is a design parameter that can be freely adjusted foroptimization of device performance; but, in reality, the supercapacitorthickness is manufacturing-limited and one cannot produce electrodes ofgood structural integrity that exceed certain thickness level. Ourstudies further indicate that this problem is even more severe withgraphene-based electrode. The instant invention solves this criticallyimportant issue associated with supercapacitors.

The presently invented process allows us to prepare graphene foam of anythickness from 10 nm to 10 mm (or even thicker). However, for practicalpurposes, we typically prepare graphene foam sheets from 5 μm to 500 μmthick. When thicker electrodes are desired, we can stack multiple sheetsof electrolyte-impregnated foam to reach essentially any reasonableelectrode thickness. By contrast, the conventional wet-dry-wet processdoes not allow for such flexibility.

Also highly significant and unexpected are the data summarized in FIG.12 for reduced graphene oxide-based EDLC supercapacitors. The cell-levelgravimetric and volumetric energy densities plotted over the achievableelectrode thickness range of the RGO-based EDLC supercapacitors (organicliquid electrolyte) prepared via the conventional method and those bythe presently invented method (oxidized graphene foam). In this figure,the gravimetric (♦) and volumetric (▴) energy density of theconventional supercapacitors are based on the highest achieved electrodetap density of approximately 0.28 g/cm³, and the gravimetric (▪) andvolumetric (x) energy density of the presently invented supercapacitorsare from those having an electrode tap density of approximately 0.95g/cm³, by no means the highest. No one else has previously reported sucha high tap density for un-treated, non-activated oxidized grapheneelectrodes.

These data indicate that the highest gravimetric energy density achievedwith RGO-based EDLC supercapacitor cells produced by the conventionalslurry coating method is approximately 15.8 Wh/kg, but those prepared bythe presently invented method exhibit a gravimetric energy density ashigh as 43.9 Wh/kg at room temperature. This is an unprecedentedly highenergy density value for EDLC supercapacitors (based on the total cellweight, not the electrode weight or active material weight alone). Evenmore impressive is the observation that the volumetric energy density ofthe presently invented supercapacitor cell is as high as 54.9 Wh/L,which is more than 10 times greater than the 4.4 Wb/L achieved by theconventional counterparts.

EXAMPLE 14 Achievable Active Material Weight Percentage in a Cell andits Effect on Electrochemical Performance of Supercapacitor Cells

Because the active material weight accounts for up to about 30% of thetotal mass of the packaged commercial supercapacitors, a factor of 30%must be used to extrapolate the energy or power densities of the devicefrom the performance data of the active material alone. Thus, the energydensity of 20 Wh/kg of activated carbon (i.e. based on the activematerial weight alone) will translate to about 6 Wh/kg of the packagedcell. However, this extrapolation is only valid for electrodes withthicknesses and densities similar to those of commercial electrodes (150μm or about 10 mg/cm² of the carbon electrode). An electrode of the sameactive material that is thinner or lighter will mean an even lowerenergy or power density based on the cell weight. Thus, it would bedesirable to produce a supercapacitor cell having a high active materialproportion. Unfortunately, it has not been previously possible toachieve an active material proportion greater than 30% by weight foractivated carbon-based supercapacitors or greater than 15% by weight forgraphene-based supercapacitors.

The presently invented method enables the supercapacitors to go wellbeyond these limits for all active materials investigated. As a matterof fact, the instant invention makes it possible to elevate the activematerial proportion above 90% if so desired; but typically from 15% to85%, more typically from 30% to 80%, even more typically from 40% to75%, and most typically from 50% to 70%. For instance, FIG. 13 shows thecell-level gravimetric energy densities plotted over the achievableactive material proportion (active material weight/total cell weight) ina graphene-carbon hybrid foam-based EDLC supercapacitor (with organicliquid electrolyte). An exceptional cell-level energy density of 46.6Wh/kg has been achieved.

In conclusion, we have successfully developed an absolutely new, novel,unexpected, and patently distinct class of highly conducting graphenefoam materials and related processes of production. These foam materialsprove to be exceptional supercapacitor electrode active materials. Thechemical composition (% of oxygen, fluorine, and other non-carbonelements), structure (crystal perfection, grain size, defect population,etc), crystal orientation, morphology, process of production, andproperties of this new class of foam materials are fundamentallydifferent and patently distinct from meso-phase pitch-derived graphitefoam, CVD graphene-derived foam, and graphene foams from hydrothermalreduction of GO, and sacrificial bead template-assisted RGO foam. Thethermal conductivity, electrical conductivity, elastic modulus, andflexural strength exhibited by the presently invented foam materials aremuch higher than what prior art foam materials.

We claim:
 1. A supercapacitor electrode comprising a solid graphene foamimpregnated with a liquid or gel electrolyte, wherein said solidgraphene foam is composed of multiple pores and pore walls, wherein thepore walls contain a 3D network of interconnected graphene planes thatform electron-conducting pathways and wherein said pore walls contain apristine graphene material having essentially zero % of non-carbonelements, or a non-pristine graphene material having 0.001% to 5% byweight of non-carbon elements wherein said non-pristine graphene isselected from graphene oxide, reduced graphene oxide, graphene fluoride,graphene chloride, graphene bromide, graphene iodide, hydrogenatedgraphene, nitrogenated graphene, chemically functionalized graphene, ora combination thereof, and said solid graphene foam, when measured in adried state without said electrolyte, has a physical density from 0.01to 1.7 g/cm³, a specific surface area from 50 to 3,300 m²/g, a thermalconductivity from 200 W/mK to 500 W/mK per unit of specific gravity, andan electrical conductivity from 2,000 S/cm to 4,000 S/cm per unit ofspecific gravity.
 2. The supercapacitor electrode of claim 1, whereinsaid pore walls contain stacked graphene planes having an inter-planespacing d₀₀₂ from 0.3354 nm to 0.40 nm as measured by X-ray diffraction.3. The supercapacitor electrode of claim 1, wherein said pore wallscontain a pristine graphene and said solid graphene foam has a densityfrom 0.1 to 1.7 g/cm³ or an average pore size from 0.5 nm to 50 nm. 4.The supercapacitor electrode of claim 1, wherein said pore walls containa non-pristine graphene material selected from the group consisting ofgraphene oxide, reduced graphene oxide, graphene fluoride, graphenechloride, graphene bromide, graphene iodide, hydrogenated graphene,nitrogenated graphene, chemically functionalized graphene, dopedgraphene, and combinations thereof, and wherein said solid graphene foamcontains a content of non-carbon elements in the range of 0.01% to 2.0%by weight.
 5. The supercapacitor electrode of claim 1, wherein saidsolid graphene foam further contains a carbon or graphite materialselected from carbon nanotubes, carbon nano-fibers, carbon fibersegments, graphite fiber segments, activated carbon, carbon blackparticles, carbon wires, natural graphite particles, needle cokeparticles, meso-carbon micro-beads, particles of a natural or artificialgraphite, expanded graphite flakes, or a combination thereof.
 6. Thesupercapacitor electrode of claim 1, wherein said solid graphene foamhas a specific surface area from 200 to 3,000 m²/g or a density from 0.1to 1.5 g/cm³.
 7. The supercapacitor electrode of claim 1, wherein saidnon-carbon elements include an element selected from oxygen, fluorine,chlorine, bromine, iodine, nitrogen, hydrogen, or boron.
 8. Thesupercapacitor electrode of claim 1, wherein said multiple pores containa redox pair partner selected from an intrinsically conductive polymer,a transition metal oxide, and/or an organic molecule, wherein said redoxpair partner is in physical or electronic contact with said graphenematerial, forming a redox pair therewith.
 9. The supercapacitorelectrode of claim 8, wherein said intrinsically conducting polymer isselected from polyaniline, polypyrrole, polythiophene, polyfuran,sulfonated polyaniline, sulfonated polypyrrole, sulfonatedpolythiophene, sulfonated polyfuran, sulfonated polyacetylene, or acombination thereof.
 10. The supercapacitor electrode of claim 1, whichis in a continuous-length roll sheet form having a thickness from 10 nmto 10 mm and a length of at least 2 meters and is produced by aroll-to-roll process.
 11. The supercapacitor electrode of claim 1,wherein said solid graphene foam has an oxygen content or non-carboncontent less than 1% by weight, and said pore walls have aninter-graphene spacing less than 0.35 nm, a thermal conductivity from250 W/mK to 500 W/mK per unit of specific gravity, and/or an electricalconductivity no less than 2,500 S/cm per unit of specific gravity. 12.The supercapacitor electrode of claim 1, wherein said graphene foam hasan oxygen content or non-carbon content less than 0.01% by weight andsaid pore walls contain stacked graphene planes having an inter-graphenespacing less than 0.34 nm, a thermal conductivity from 300 W/mK to 500W/mK per unit of specific gravity, and/or an electrical conductivity noless than 3,000 S/cm to 4,000 S/cm per unit of specific gravity.
 13. Thesupercapacitor electrode of claim 1, wherein said graphene foam has anoxygen content or non-carbon content no greater than 0.1% by weight andsaid pore walls contain stacked graphene planes having an inter-graphenespacing less than 0.336 nm, a mosaic spread value no greater than 0.7, athermal conductivity from 350 W/mK to 500 W/mK per unit of specificgravity, and/or an electrical conductivity no less than 3,500 S/cm to4,000 S/cm per unit of specific gravity.
 14. The supercapacitorelectrode of claim 1, wherein said graphene foam has pore wallscontaining stacked graphene planes having an inter-graphene spacing lessthan 0.336 nm, a mosaic spread value no greater than 0.4, and/or athermal conductivity from 400 W/mK to 500 W/mK per unit of specificgravity.
 15. The supercapacitor electrode of claim 1, wherein the porewalls contain stacked graphene planes having an inter-graphene spacingless than 0.337 nm and a mosaic spread value less than 1.0.
 16. Thesupercapacitor electrode of claim 1, wherein the solid graphene foamexhibits a degree of graphitization no less than 80% and/or a mosaicspread value less than 0.4.
 17. The supercapacitor electrode of claim 1,wherein said pore walls contain a 3D network of interconnected grapheneplanes.
 18. The supercapacitor electrode of claim 1, wherein said solidgraphene foam contains pores having a pore size from 0.5 nm to 100 nm,measured in a dried state without said electrolyte.
 19. Thesupercapacitor electrode of claim 1, wherein said liquid electrolytecontains an aqueous electrolyte, organic electrolyte, ionic liquidelectrolyte, or a mixture of an organic and an ionic liquid electrolyte.20. A supercapacitor comprising an anode, a porous separator-electrolytelayer or electrolyte-permeable membrane, and a cathode, wherein eitheror both of said anode and said cathode contains the electrode ofclaim
 1. 21. The supercapacitor of claim 20, which is a lithium-ioncapacitor or sodium-ion capacitor having a cathode containing theelectrode of claim 1 and an anode containing a pre-lithiated anodeactive material or a pre-sodiated anode active material.